"Perhaps you're still generating the same amount of ideas it's just that you're brain is better at filtering out the bad/impractical ones at an early stage?"

Definitely one factor.

"The obvious downside being that maybe some of the assumptions you hold based on experience may not apply anymore."

For technical things I don't think so, as I keep up. The danger I am seeing, is rather that I might dismiss ideas, because I failed at implementing similar ideas in the past because I lacked the skills/ressources. I know some old people developed this habit towards the young. Since they failed at it, how could those stupid young people succeed ... so making sure they will not.