[content warning: discussion of aging as a negative thing]
If you are currently young-looking and would like to remain so, the single best thing you can do is WEAR SUNSCREEN. UV exposure is responsible for approximately 80% of facial aging.
Make sure your sunscreen protects against both UVA and UVB rays, as many sunscreens do not. UVA rays tend to be aging, while UVB rays tend to cause sunburn. If you live in Asia or Europe, you’re in luck! Your sunscreen includes ratings of how well sunscreen protects against UVA (PA in Asia, PPD in Europe). If you live in the US, you can look for sunscreens which offer broad-spectrum protection (although the use of the term is not regulated) or consult a website like this one. You can also look for the Seal of Approval from the Skin Cancer Foundation.
If you have sensory objections to sunscreen, experimenting with different kinds of sunscreens is worth trying: sprayable sunscreens and physical sunscreens have a different texture. [ETA: Physical sunscreens like zinc oxide provide a physical barrier between the skin and the UV rays, while chemical sunscreens transform UV rays into heat.] Physical sunscreens may also be better for aging, as chemical sunscreens may lead to the production of free radicals, which cause aging (however, even chemical sunscreen is better than none at all).
Apply sunscreen each morning, and reapply every two hours when you’re outdoors. Apply LOTS; most people don’t apply enough sunscreen. Apply sunscreen to your neck as well as your face: you don’t want a youthful face and an aging neck. Wash off your sunscreen each night with a gentle facial cleanser, especially if you’re prone to acne. Supplement with vitamin D, because daily sunscreen use WILL make you vitamin-D deficient (especially if you’re vegan).
Reducing your sun exposure also helps prevent damage. You can reduce your sun exposure by staying indoors (especially between 10am and 4pm), covering as much skin as you can, and using a parasol.
Finally, remember that you are competing with a bunch of people who fry themselves in tanning beds and refuse to apply sunscreen before they get that ‘healthy tan’. There is no need to go overboard. If, personally, you can’t be arsed to reapply every two hours, or use a sunscreen when you’re going to stay inside, or adopt the agoraphobic Victorian maiden lifestyle, you’re still probably going to be doing better than the people who get sunburned every summer. It’s all about your own personal desires.