David may have had cosmetic makeovers etc + choices of clothes, hair etc also being a contributing factor to looking young.
In broad strokes the key to stay as youthful as possible seem to be the following.
1. Calorie restriction with optimal nutrition, as much as you can manage, with maybe a BMI of 16 as a lower limit or so.
2.Keeping glucose levels low(also a part in the CR area)
3. Avoiding fried food,processed,transfat,things like that.
4.Avoiding the obvious things like smoking,alcohol,drugs,toxic substances,pollution etc...
5.Avoiding psychological stress which may create too much cortisole in the body which is harmful and aging.
6.Avoiding the sun as much as possible,using sunscreen strongly protecting against UVA and UVB .
7.Using tretinoin daily, and topical antioxidants like vitamin C etc...
8.Getting enough sleep.
9. Exercising lightly/moderately.
10.Behave like you are young,exercise your brain. Wear clothes associated with youth.
This sums up the current advice as for how to stay young, doesn't it?
Time to adopt such a lifestyle then....