Twenty five year olds can have bad backs and your HR department knows your real age if anyone questions it (and you could always retroactively claim you were joking on your first day and thought everyone was in on the obvious deception).
Assuming it isn't an inherent job duty, ask not to lift heavy things and if someone objects then get a note from your doctor.
Nosey people can get fucked - we all have the freedom to choose what parts of our private life we share with coworkers and what we don't so you have no obligation to disclose your age (outside of anything relevant to payroll/taxes/insurance enrollment).
There is no need to change anything. Just don't let yourself be pressured into work you're not capable of doing. And if anyone questions your marital status or what-not... They're an asshole and can get fucked.
Edit to add:
After some discussion in the other thread, I think it's pretty regrettable to outright lie about your age - if you change jobs again, I'd really suggest just refusing to answer and deflecting the question. Telling such a direct lie likely makes some of your coworkers uncomfortable.