not as young as (one) used to be
No longer as physically fit or able as one might have been in one's youth. You shouldn't try to tackle such demanding hikes—you're not as young as you used to be. I think it's time I started cutting down on my drinking. I'm not as young as I used to be, after all.
not as young as one used to be
Fig. getting old. Aunt Lila isn't as young as she used to be. She can't take a lot of trips anymore. Don't walk so fast! I'm not as young as I used to be. It takes me awhile to catch up.