I'm 19 and started my transition just before entering high school. My mom has always been so supportive of me since the days I was 5 wanting to dress up, play with make up, ect. My dad didn't feel the same way sadly and there is still a lot of tension between the both of us.
My mom and dad divorced when I was in Grade 5 / 6 which also played a great role in my mom allowing me to transition at such a young age... If my dad was still in the picture at that time I'm sure it would of been a very different story for me.
On summer break before entering my first year of high school my mom took me out for lunch and we had a LONG talk about everything. She realized what I was feeling was much more than a phase and told me the option was there if I truly believed this was who I was meant to be... Obviously I didn't second guess and have no regrets.
In my mid teens my mom and dad decided to work things out and are currently back together... This created hostility within my family but I try to remain strong and positive for the most part. I still love my dad even though he has openly protested his shame, anger and hate for me... Why I stand up for him to my mom when they fight about my life and everything that's happened is far beyond my comprehension... I have hope he will come around eventually... He never kicked me out or left my mom so I have a feeling he wants to love and accept me but the way he was raised has him a bit torn and conflicted... I may be naïve and giving him to much credit but, I try to keep a positive outlook on life.
Although, I can't speak on any other alternate reality I could of lived but, I feel if I was over 40 years of age and was finally able to be the person I felt I was meant to be - I wouldn't hesitate one second. I was fortunate enough to have the right events occur in my life for this to be possible for me and I'm truly thankful for it.
Don't let anything hold you back! You only get one life to live - Live it the way you were truly meant to live it. Your happiness deserves it

*EDIT*
I forgot to mention - I agree the most ideal age to start is around your late teen / early 20s for many reasons... I was young and surrounded by a bunch of kids who couldn't grasp the idea of who I really was... I was called every name you could possibly think of, jumped and beaten up many times, had things stolen from me, introduced to people with rotten personalities who were 2 faced and made me the center of an ongoing joke. It was rough. I cried many times and wished I could of been what society deemed 'normal' to just fit in for once. I made friends but I could always count them on one finger which is good and bad all in itself. I understood from a young age I was different from others so there were many times my good friends would go to parties, events, dances where I wasn't aloud to go or accepted to join in based on who I was. I didn't hold my friends responsible for going and leaving me behind for social gatherings I really wanted to join in on also. The few times I actually did attend hang outs after school I was just a target for people to get amusement out of. No matter what age you are it's going to be tough no matter what - You just have to be strong and try to understand why people don't like you the best way you can.