Definitely a girl can transition after age 20...yikes... of course YES.
I'm in my 50's. This is a lot more years of testosterone poisoning to overcome. Yet, I have worked as an actress "extra" (for transportation expense reimbursement only but yeah that's real money isn't it?

) on nationwide television with millions of TV viewers in Europe as well as actress "extra" on Internet TV with over 170,000 youtube views. And my photos to millions of nationwide newspaper readers.
Now, certainly it would have been much better for me to have transitioned to JENNIFER when I was 21 years old. Likely I could have been a model. Oh, well, but still better late than never. There was no Internet then with trans info and things were less socially accepted then. So I didn't know how to deal with my dysphoria issues at that time.
One downside that later transitioners should be aware of that can be difficult to handle emotionally is intentional misgendering and bullying due primarily to physical appearance. This can come from cis (non-trans) people and from other trans girls. Bullying and intentional misgendering is more deplorable coming from other trans girls since they too are a very small minority and might have faced many of the same bullying issues earlier in their transitions. I mean, at that time I still had to use a wig (I never use a wig now) and the wig hairline was sometimes a little obvious. Anyway, I kept working at passing as even more cis beautiful and now I feel this has been accomplished. I rarely get misgendered now in face-to-face real life. But last year I almost did suicide because of so much bullying against my physical appearance from both cis (non-trans) people and some trans girls. In real life now, most people who didn't know me before I transitioned to JENNIFER just take me as a beautiful older woman.
Now, in real life face-to-face I just "go stealth" meaning I live 24 /7 as full-time, legally and medically hormonally Jennifer and I don't even tell people I am a trans girl unless absolutely necessary.