Also an lj-friend emails me to point out this New Yorker article, showing that poetry is not the preserve of young upstarts, and that many of the most beloved poems were written by the over forties.
I think an emphasis on youth favours rich young men who are more likely to have spare time and confidence.
ETA - also H points out that some poets like Keats and Plath and the WW1 poets wrote only in their youth because they died young, not from any falling away. Who knows how even greater they might have become?