Sexuality and gender identity are tricky. It's not unusual at all to find our you're gay after 20. Some people don't figure it out until they are in their winter years, so in your 20s is still pretty early on in the grand scheme of things!
I think that discovering this aspect of yourself can come at any point in life, regardless of what anybody tells you. I came out to my mother in my second year of high school and I was met with reluctance because apparently, it is something that 'you can tell' at an early age. The moment at which I found out I was bisexual didn't make me straight or any less valid.
You don't have to know it at any time. Sometimes you just don't really experiment early or try anything new, so you don't know. Or you just don't really notice it too much. But your sexuality is yours and yours alone. Its not like someone can say "nope, you've only been gay for a month so it doesn't count." It doesn't matter when, or how.
There's no specific age for discovering your sexuality - it happens earlier for some and later for others, and it's ok anyway! If you feel like your sexuality is different than what you thought, then you're free to identify with whatever describes you best. You're not "less gay" if it takes you more time than others to find out, everyone has a different path, your identity is valid!
Realising your sexuality does not always happen at an early age, there are lots of people who take their time in figuring their sexuality out. There can be enough stress in realising this, try not to worry too much on how long it has taken. Be kind to yourself and build that confidence in who you are, know you have lots of people in your corner supporting you! :) There are lots of people who take their time, they have experiences in their lives that help them realise who they are as a person. There is not fault in understanding your sexuality at an older age.