As with most things, there are upsides and downsides to starting young. Here are the ones I could think of, from my experience.
Pro #1: Lots of Awards
This is one very obvious benefit - there are so many competitions designed specifically for under-18s or other young age groups, and they're pretty big deals - international science fairs, for example, come with prizes like $50,000 or $75,000 for the winners. If you're a young person with talent and/or work ethic, there are bound to be tons of competitions for you to enter, from school-level to international-level. With adults, the competitions are a lot rarer, harder to access and of an extremely high standard. That's not to say competitions for young people aren't difficult, but since there are a lot more of them, there are going to be some that are pretty easy.
Almost all of my awards are based in part on my age - they're not going to give a forty-year-old "NI Young Scientist of the Year", or call them a "Global Youth Leader in Nanotechnology".
Con #1: Condescension
Condescension really annoys me, and unfortunately it's very common when you do things with some special condition e.g. you're young, or female, or a minority race. You get comments like "This is great ... for a teenager". Now, I don't know how you feel about it, but I'd rather be known for doing work that's great on its own.
That's why I love online freelancing, which I do occasionally by selling articles to various small companies and outlets - nobody knows or asks what age I am or what educational qualifications I have. All that matters is the merit of my work, which makes my five-star reviews from satisfied paying customers even better, because I know it's not being good "for a teenager/girl" - it's because it's good work, simple as that.
Pro #2: Community
Because you're doing something fairly unusual, there aren't too many other young people in your area doing it - so you get to know everyone fairly quickly. It becomes a very small world when you see the same young people at conferences all the time, and it's nice, because if there are some young people there you'll definitely have people to talk to. My sorta-professional sphere lately has been the Irish teenage tech/science scene, and honestly we pretty much all know each other, at least from Twitter.
Con #2: Balancing School
This one is a pain. As a young person, in most cases you can't just decide to go full-time on your project the way you could if you were an adult, because you're, y'know, legally obligated to get an education. Now, I absolutely loved school, but it's a hassle when you're up in a university lab one day a week, spending your free time writing and reviewing the scientific lit and emailing (so much emailing) and working on your website and all the other things you're doing, while also balancing the workload of school and teachers who Do Not Like it when you miss their class, especially in an exam year.
Obviously everyone has to balance things with school, but when it's adult-y stuff (a) work often has to be done during school hours, because those double as normal adult working hours (b) it kinda changes your perspective on school. Like, when I was getting paid to write dozens of articles on all sorts of interesting and not-so-interesting topics, it definitely made me more reluctant to write about boring things for English homework, for free. My teacher might give me a C on an essay because I didn't use four adjectives for every noun (???) and I'd be very tempted to show that people are happy to actually pay real money for me to write useful stuff for them instead of needless decorative language. Like, yes, I do have a large vocabulary, and I could absolutely pack my sentences with fancy adjectives and flowery adverbs if I wanted to. But that doesn't make the writing clear, readable and useful, so why bother?
Pro #3: Fewer Obligations
This is the other side of the coin of Con #2 above. While school is a big obligation, at least as a teenager your free time is mostly free (then again, adults usually don't get homework...). You might have to do some housework, but at least you don't have to worry about providing for children, or running errands, or filling up your petrol tank, or buying groceries, or getting an NCT. People usually aren't relying on you. So you get time to explore your interests and try out different things you might like without worrying too much about how this is going to provide for a family.
Con #3: Media Mishaps
I am no stranger to media mishaps, let me tell you. I've done quite a few interviews and oh man - while I'm not exactly great at them yet, I've improved so much from how I started out, being interviewed in 2014 as a 15-year-old by radio people and judges at my first Young Scientist. I have said so many embarrassing things, and occasionally I remember them and cringe, so much cringe.
So part of it is interview experience, and part of it is just life experience. I think if I was older when I started to do them I wouldn't have said so many embarrassing things because I'd have more practice in being diplomatic, in simplifying things into (accurate) soundbites and in coming up with things on the fly when questions are flung at me out of left field. I imagine the effect must be much more pronounced for people who (a) started younger (b) are much more successful (not hard).
Then again, I'm glad I now have those skills. Trial by fire, I suppose - taking a grilling at science fairs (being judged by 11 judges in about two hours at Sentinus Young Innovators (NI Young Scientist, for example) makes you learn a ton of skills fast - how to stay bouncy when you're exhausted, how to think on your feet, how to explain things so they're understandable to people from many different fields. I also had to learn the difference between science judging interviews and more mainstream media interviews, where they really don't want the nitty-gritty details of your procedure. It's a tricky balance, expressing science as simply as possible - but no simpler (to paraphrase Einstein).
Pro #4: Support & Generosity
There are many kind souls out there who'll see a budding young scientist or writer or artist or musician or whatever and give them help out of the kindness of their hearts, with no prospect of immediate reward, and I am so grateful for those people, as are, I'm sure, all the other people who do this sort of thing. My lab supervisors, for example, who made most of this possible - Sinéad, Nina, Prof Duesberg, Mary and Éilis in ASIN; Yuri, AJ, Laura, Adriele and more in St. James'; Margaret, Colm .... you know the drill. Teachers, too - students have a readymade collection of knowledge fonts whose job it is to help them. How lucky!
Basically, people seem to see young passionate people as especially worthy of help, so that's a huge benefit to starting young. If you can get people to listen to you and take you seriously, they can be very generous. (Side note: I'm really looking forward to the point in my career where I can be the helper, instead of just the helped -- it's already happening to a small extent, but yeah, looking forward to it.)
Con #4: Disbelief
It can be very, very difficult to get people to believe in you. Even once you've managed to find help and you've done the work, there'll be people who refuse to people you actually did the work yourself - like suspecting your parents wrote the report book you toiled over just because it's well-written, or nobody believing you came up with your own ideas. Like sure, everyone gets guidance and that's a great thing, and I'm very grateful for those who've mentored and guided me -- but you still have to do the bulk of the work yourself, and when people don't believe you did that it's very disheartening. It can also be very difficult to access facilities that an adult would find a lot easier, because as a teenager you probably don't have a college degree, and besides, many places require you to be 18+ for insurance reasons.
Pro #5: Passion
This is the biggest thing of all, and this is why I and presumably many others do it despite the cons.
When you start young, you get a headstart on exploring your passion. It's something you love - why wouldn't you start as soon as possible? It means more time spent developing yourself and your interests and doing something you get fulfillment out of.
Without this final factor, the awards and community and all the other pros would be nice - but this is the crucial one. Passion is what outbalances all the cons. Sure, you probably don't have your whole life figured out yet - but if you've found something you're good at and can bring you closer to your dreams, starting early is a no-brainer.