I disagree with your assessment. For starters, excepting Barbara Wright, I don't think there was a single Classic Who female companion over thirty. Several were supposed to be under 20 (i.e. Vicky, Susan, Ace - their actresses weren't teenagers anymore, but the characters were supposed to be), and most were supposed to be in their early 20s. So it would be utterly hypocritical to lament this as a new development. (Rather than lament this as a tv tendency in general, which is what I did.) On the contrary: I dare say Catherine Tate would not have been cast as a companion during the Classic Who heydays, let alone Lindsay Duncan.

As for New Who ignoring older actors, one look at the list of guest stars of the last four years would belie that assessment. Zoe Wannamaker, Simon Callow, Derek Jacobi anyone?