There needs to be made a distinction.
Sex never expires, not even for 60yo grandpas. In that sense, it's never too late. We crave sex until we die.
However that is just the pleasure part. It feels good, but that's not what this is about. You can be 35yo virgin and nut in a prostitute but everybody agrees that's worthless.
It's about the confidence and self-esteem that comes with it. Having sex in your teens will set you up for your later life because it's an amazing boost towards a positive feedback loop. Take two completely identical 16yo boys and have one experience a relationship and keep the other a virgin into his 20s and their entire lives will play out dramatically different.
So to rephrase your question: When is it too late for sex to give you that essential boost in confidence? I reckon at ~18yo when you graduate HS. If you're a virgin by then, that's the first massive dip in confidence, however you can still ascend in college and make up for it and become "normal". After college (~22yo) it's safe to say you missed out on this effect. Your youth is over, you are a fully grown man.
I am a believer in teen love. I know guys who have basically nothing going for them but they are still all-around happy people because they lucked out and had a GF either currently or some years ago. That shit makes an incredible difference.