I believe the average for transition is going down. Older transitioners don't have as much chances to pass as well as younger ones... Especially in the earlier stages. More older people = less passability, in theory.
I see many 30-40-year-olds around here who end up passing very well... But in their first 3-6 months, many weren't even close to passing, whilst a similar proportion of their peers half as old as them were already very close to, or already passing.
Plus, I assume that people visiting a clinic often aren't very far into transition. The further in you are, the less often you visit the clinic, right?