In 9.4, not really. In 9.5 there's a function mxid_age() that gives you the age of a multixact, so you'd grab the oldest from pg_database.datminmxid and compute the age of that one. Going from the oldest multi to the oldest offset cannot be done without an additional function, however. It's much easier to keep track of the oldest file in $PGDATA/pg_multixact/members/; what you really need to care about is the size of the "hole" between the newest and the oldest files there. Once newest starts to stomp on oldest, you're screwed.
Thank you Alvaro. We're beginning to plan our rollout of 9.5.
