I remember a sutta from the Pali Canon where the Buddha comments to his walking partner that “old age sucks”. He said it in a personal way, the way an old man in our time getting out of a chair might say it to a companion. He wasn’t lecturing.
I don’t remember a sutta in which the Buddha complains about his body aging, only those in which he states it as a fact. Maybe you’re thinking of something like DN 16:
I’m now old, elderly and senior. I’m advanced in years and have reached the final stage of life. I’m currently eighty years old. Just as a decrepit cart keeps going by relying on straps, in the same way, the Realized One’s body keeps going by relying on straps, or so you’d think.