Oh boy. This thread is sure to see some FI definition arguments discussions.
Taking what seems like the OP's definition, I'm sure there are some 18 year olds who went on SSI disability for life.
Possibly before that, if their parents got payments for them that covered all the kids' expenses?
As far as people FIRE'd via saving 25x their expenses, there's more than a few in their early to mid-20s. Sustainability is a question if their expenses are very (unsustainably?) low.
I'm sure there's plenty of tech entrepreneurs or cryptocoin holders who hit FI very young too, though again, sustainability is a question if they aren't purposefully looking at FI, as they may spend more than would be safe, or be invested in too risky of assets.
Age doesn't matter so much. For getting to FIRE, it's about how old you are when you find it, and your baggage when you do.
If you find the concept at 20 with very little debt, late 20s to mid-30s is totally feasible. If you find it at 45 with tons of consumer debt, you might be happy with an early retirement at 62.
We're all on our own journey. Looking to others doesn't help, unless it's for inspiration and ideas.
:)