For *appearing* younger, I believe whitening teeth is the most important one. White teeth also makes your teeth appear straighter than they may be. Yellowed teeth is incredibly aging and even if they're straight, so long as they are yellow, jacked-up white teeth is more attractive (and youthful) than straight yellow teeth.
I also think too large of teeth (where they look like dentures), and too small of teeth (where they appear like nubs or appear that the person doesn't have any teeth), is very aging.
I think the second most important for appearing younger is covering or lightening sun spots and discoloration.
And taking care of facial hair. Whiskers and any noticeable facial hair are aging.
These three I believe are the most important "cosmetic" things for *appearing* younger.
I think a woman can look drastically younger by focusing on just these things, far more than if her focus was on concealing her undereye bags, grey hair, and wrinkles. Though grey roots (or roots lighter than than the rest of the hair, whether on older women or younger women) is very aging too, imo.
Great question.