Welcome La Ran. Happy you can still ride a mountain bike. I'm 51 now, but in 1991 a car hit me, high speed on a motorcycle... in my late twenties, rode an 18 speed mountain bike for a while; actually, I only used 6 gears - largest chain ring and 6 cog cassette (most gears ratios are duplicated anyway, but more gears sounds better for market). Had another motorcycle accident in 1999, and didn't ride again until 2016. Originally with a 24" 7 speed adult tricycle. Couldn't safely ride a 29er 21 speed mountain bike on the street, but in a parking lot discovered it has much more speed with less effort peddling opposed to the adult tricycle. Tried stabilizer wheels on the 29er, didn't fit good, but rode it anyway. One day a jump between stabilizer wheels caused another crash, but this time broke my hip. Couldn't walk and had total hip replacement surgery. Started riding adult tricycles again, 4,569 miles in 2018 broke in my new hip.
Indeed, that is remarkable to still ride a mountain bike. my brain injury has progressed and prevents me.