I was 15 when I learnt and it took me only 30 minutes but I had one advantage: I already knew how to drive shift-stick vehicles all by myself in the backroads of Houston,Texas. My uncle taught me in a 1982 Kawasaki.
It amazed me that my brother-in-law, a kid of 13, learnt how to completely master a motorcycle without even knowing how to drive an automatic car and being clueless as to what a "clutch" or "gears" was. We started at 3:30pm and by 5:30pm he was already riding the motorcycle all by himself, making u-turns on the road, stopping, starting, slowing down, all of this without stalling the bike or crashing.
I also tried to teach my 9 and 11 year old nieces but their feet could not touch the brake/shift peg but I was riding behind them without interferring and they rode the motorcycle all over the place as long as I had placed the bike on 2nd gear for them, I know that they could have ridden it by themselves if they could only reach the pegs because I also taught them how to change gears and in theory they knew how to do it, this in just one afternoon.
This is why in Humber College I had not sympathy for full-grown adults who crashed the bikes several times during our M2 tests, c'mon are you kiding me, they have been practicing for days before they go to Humber haven't they?