If she's saying that she gets younger every time she sees her I'm assuming it means they don't / didn't see each other quite often. It's like something you tell a family member you haven't seen in several years.
Yes, they obviously don't see each other monthly or anything. But let's get numerical.
The play was November 2010. Studies show that David, Tea, Gillian, and possibly Mark had dinner together in NYC in October 2009.
Given Tea's total absence from the IWTB premieres in July 2008 (and the fact that she was moving house at the time and probably already separated from DD), they didn't see each other then. Tea did go to Whistler (Vancouver) with the kids while they shot the movie in early 2008, but whether they visited the set and saw Gillian is anyone's guess.
The last time before then would almost certainly have been 2002.
I'm just trying to be thorough.