I'm about the same age, I've been taking metformin about seven years and it seems to still work for me, if I miss a dose or about once a year I try cutting back the dose and sure enough my numbers blip back up.

I do have increasing trouble keeping my daily blood glucose numbers down, but that's not the metformin's fault, I know it's for two (or more) reasons. First, I cheat more on my diet, LOL. When I stick to my carb limits my blood glucose is fine. Well, mostly fine. I think my carb processing capacity is slowly falling. Cheats that I used to get away with, not so much anymore, so my BG numbers go up and may take a day or two to come back down. So that's the second reason. Or, just call it age.

So I keep revising my carb plan and I try to cut back on the cheating.

ps - it's not just carb cheating, it's fat cheating as well, red meat and milk fat seem to raise my numbers, even if I stay within my carb limits, probably facilitate insulin resistance but exactly how I don't really know.