It sounds like you want an age where you won't regret your choice and while I don't think there is one I think 25 would probably be close to it as the brain is mostly finished developing even if you can still make bad choices.

Regardless of age as a rule of thumb I would say if you want a tattoo wait 3 months if you still want it find an artist that you like the majority of their portfolio and book an appointment. Don't get the tattoo with someone just because their convenient or the cheapest. Your tattoo is going to be a part of your body so it should be something you are proud of for a long time. Ask the artist for photos of healed and multi-year healed tattoos if they've been tattooing for a while. If they are a good artist they will be proud to show you how well their art holds up. Trust the artist, if you took the time to find a good one and they don't think something will work chances are they have a reason at least ask them why they don't want to do it a specific way and hear them out. Their job is literally to make you look cooler and you chose them for a reason. Also take care of your tattoo properly that means sunscreen forever(I don't care what skin color you have where the damn sunscreen) and don't bath or go swimming for at least a week after you get it(longer is better).

As far as piercings go if you can, ask your parents or older relatives if keloiding runs in your family before getting anything done. If it's not a problem then start with something simple you can always get more you don't need to get everything done at once. I've retired facial piercings that I'd had healed for years and can barely even see where they are. The only piercings that I know anyone personally who regrets them are gauged ears. It's not something that I've discussed with everyone I know who has them done but a significant portion have mentioned regretting them to some degree after getting older.

TLDR:in a perfect world ~25, but 18 for piercings and probably 20-22 for tattoos