We just got my 25 percentile 8 yo a 20" bike; the height on the 24" isn't what worried me as much as the reach to the handlebars. But if my kid was as big as yours, I'd probably go with the 24".

My 8 year old (also tall for his age) rides a 24 inch. Might be worth skipping the 20 inch, especially since your DC will be taller by spring/ summer when he'll be riding it the most.

My 55" 9 year old is on a 24 inch bike

It's easy, OP. Sitting on his bike, your son should have both feet barely touching the ground at the same time (shoe tips, not heels). If he can do this in the lowest seat position on the 24" bike, he's big enough for one.

My 5th percentile 8.5 year old is still on his 20" bike. I suspect it will last him 6 more months, if not a full year.

My 10-year-old (plus a few months) is still on his 20" bike, and he's average (50-60 percentile) height. We've had to move the seat up so he can pedal properly, but it'll last him through the spring (we hope). This summer we'll get him a new bike, and give his to his younger brother.

My tall son just got a new bike to replace his 20" bike for his 9th birthday. The old one was pretty small looking for at least the past year, would say it would not last too long for a tall 8 year old.

20" bike sounds small for an 8 year old at the 95th percentile. My 11 year old at the first percentile, who is probably shorter than your 8 year old, is on a 24 inch bike.

He might need a 24.
I recommend having him measured at a real bike store.

just depends on how fast he grows. the seat is adjustable so that will accommodate some growth

I know it's a silly question. I want to get my son a bike for his birthday. He sizes at a 20" bike.

How long will it last? How many years might I get out of it.

He is tall for his age, and is at the 95% percentile when measured annually.