Not just experience, but time to learn and absorb a lot more things. Just loading information into your brain isn't the same as combining and synthesizing it with other information. And the more information you have, the more ways you can combine it. It just takes time to get all that stuff in there.

If you look at it logically, assuming a constant motivation, an engineer that was pretty good at 25 will be astonishingly good at 50, because she would still retain all (or at least most) of the skill and information she had at 25, plus EVERYTHING she's learned in 25 years.