I vote for age. I think I'm facing the same problem in the last couple of years (and also in switching from tuba to bass trombone as my primary instrument). And I'm 10 years older than you. I never noticed the problem with the tuba (including the past year). But my tendency is to "naturally" play a bit flat now on euph or bass trombone -- exacerbated by using a large mouthpiece on the large bore baritone instruments. If I move to a smaller mouthpiece, the pitch comes up. It's really irritating. I'm pretty sure I could fix it with embouchure exercises, but have been too lazy to do it that way, especially since on the trombone it's easy to fix dynamically with the hand slide. On euph, it's REALLY irritating. I don't think that chasing the problem with mouthpiece changes will be an effective way to go unless you're willing to change your overall sound.
I have noticed (in community bands) that the primary problem with older players (okay ... elderly and quasi-elderly -- the French use the term "les anciens" ) is a tendency to play off pitch (sometimes a LOT off pitch). This can be a result of either hearing loss or embouchure loss, and it can be either in the sharp direction or the flat direction. Muscle tone in the embouchure degrades and results in flatness -- except that in the high range people end up playing sharp because they're straining to get the note out. So they end up playing sharp and flat. And then hearing loss intrudes and they can't tell they're out of tune. In the band I'm currently playing in, this problem is not very pronounced (lower average age) -- except in the trumpet section -- which (surprise) is dominated by les anciens. Alor, je suis un ancien. C'est tragique.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)