Here's something that struck me:
The average first-solo (say, 15 hours) on a plane seems about the same (or maybe even shorter? ) than the average "first solo" while learning to drive a car.
To me that was non-intuitive; an aircraft seems more complex and also more outside most people's typical range of experiences. Also, you can't just "pull over", the controls are in 3D rather than 2D etc.
Maybe one compensating factor is that you don't have to worry about as many third parties flying around you as in driving.
Oh, also, when I got my "rating" at the university sailing club's 30 foot yacht it took me a bit more than 15 hours too.