It's not the age, it's the marital status that affects the productivity the most.
[1] http://www.jrhenry.net/johnrhenry/erdos.htm [2] http://www.nndb.com/people/401/000032305/
One of the times it was specifically brought up in reference to a known mathematician. The takeaway being that getting in a relationship, or even married, automatically disqualifies you from achieving anything of great importance in the future....made me pretty sad that it's such a common pattern of thought.
But then, I started my family early. I didn't have much of an established pattern of productivity before. So my baseline comparison may be way off.
As Hume would point out, is is not ought.