crotch rockets are more maneuverable, lighter, have better suspension, and are more aerodynamic.. etc. besides, alot of younger guys, as you said, learned on smaller dirtbikes, and worked their way up, rather than jumping right on the big bikes, at middle age.
personally, i'll take my standard VJMCs over either.
Whatza VJMC?
Add to that, young'uns have lots better reflexes than us old duffers.