At 7 weeks or 7 months I would personally hold off. How would you manage to keep your baby from pulling them out and then choking on them, would be one of my concerns.
But I put babies and toddlers into baths that they didn't want to have, brushed tangles out of hair, kept on grocery shopping despite howls of rage from family members strapped into trolley seats. There's a lot that I did just because it was convenient for me or because I wanted DCs to look or smell a certain way. Dirt is not going to kill you, and I could probably have shopped another time. Unbrushed hair would definitely have been the DDs' choice.
I don't think there is that much of a difference. The DDs were not competent to make the decision to have their ears pierced at 7. I essentially took on that responsibility when I agreed to their wish to have it done. I didn't let them watch some of the movies, etc., that they wanted to see at age 7 or even older, so clearly I felt ear piercing was less harmful than watching something unsuitable. It boiled down to the fact that I thought they would look nice with pierced ears and felt it was a reasonably harmless thing to let them do.
One of them wanted a tattoo at age 16 but I wouldn't let her. She could have got it done at a dodgy place that wouldn't have requested ID - it's illegal here for anyone under 18 without parental consent. I took the chance that she would be sensible. Part of my objection to the tattoo idea is that it is permanent unlike an ear piercing, and also I do not think tattoos are attractive, especially as you age and parts of you sag (apologies to those who have them and like them). I did not give permission to another DD who wanted cartilege piercings in her teen years. (Needed here for those under 18 again). This was because of concern over healing and also concern over future first impressions at interviews, etc., though I am aware I am being a fuddy duddy here.
A good deal of my decision making was therefore informed by concern over aesthetics.