Garnier Vitamin C Serum and their masks are notorious for causing reactions, have a quick check of reviews on Google and you'll see what I mean. Most forms of Vitamin C found in skincare are ascorbic acid of varying concentrations; like other acids, ascorbic acid carries the risk of irritating skin.
Age doesn't really matter, if you're an adult who does skincare you can (probably) use it. I've personally seen the effects of tret and rets be the most helpful on those who have facial scarring, pockmarks, wrinkles, thin/papery skin, etc etc... Otherwise, trets and rets are mostly just a rip-off if you're a Normal Person. If your skin is fine and you just want something to keep you looking glowing and youthful, standard cold cream like nivea or something will do the trick just fine. I've seen girls hawking sunscreen but its basically just the same stuff as cold cream.
Retonoids should not be used on thinning skin because that skin is already fragile and overuse of retinoids can cause skin thinning. Imagine the harm it could do to an already thinned out area such as around the eyes.
Sunscreen is specifically designed to protect skin from the sun, it's literally in the name. Cold cream can be used after you've already been sunburnt to soothe, it does fuck all to prevent UV damage.