I have recently been exploring/enjoying some Mencia. I understand it ages well. Hoping to find, try, taste some older vintages. Any ideas where I could find some (buy)? I am in USA!
Not much of a market for it, this is why people need to cellar wines for their own idiosyncratic tastes. How ‘old’ is old to you?
I understand higher quality can age 20+. I would settle for 10+!
Believe me my cellar has its own idiosyncratic personality!
It’s not cheap, but Wine-Searcher is showing three pre-2010 vintages of the Palacios “Las Lamas” available at various shops. I drank the 2007 this past December and it was outstanding.
From my limited tasting of old(er) mencias, which the ones I’ve had were from 8-12 years old, I find the wines mellow but do not necessarily gain much complexity, in a way like petite sirah or tannat witout the heavy tannins. I enjoy them, but question whether aging is the appropriate response. Their worth trying, but I doubt they will be transformative the way some other wines can be.
Bravo! 
Personally, I’ve been on a Cesanese di Piglio education program myself, and have been blessed with one outstanding one from the 60s. I’ve got another from the 70s waiting for the right night. So, go for it, I say!