Assuming mark ever got to that age.
she can direct her attention and purposefully alter others during that healing period so if mark were ever to get seriously injured or crippled, but survive, she would likely return him to peak health and that might put him backwards in age too.
Theres definitely some limit but they age slower as they get older and Oliver is likely proof that the aging is not a linear increase but exponential.
Maybe if they reach 100 physically they age at 1 year per million. I mean I doubt it but it’s technically possible, and if viltrum was always so violent maybe even they don’t know the limits of their age because they die in battle.
Again I doubt it it’s just technically possible and the fact that eve can just make him young again is a bigger reason to suspect the couple are functionally immortal if they so choose.