If skin is kept in very good contidion, using sun screen, moisturized, fed anti-oxidants, vitamin-c, or even the ceramides and peptides from you are 20 until 40 that should in theory give the best foundation for great skin. Even if you stop using one of your key active ingredients that should not ruin the results you have gained. Still protection from the sun is one of the most imortant things.
What many experts agree on is that skin can up to a degree recover from pre-aged conditions like wrinkles and pigmentaion problems caused by too much sun, smoking, an unhealthy diet and all that. The results gained from stopping, smoking, full working days at the beach, not sun screens, and a very poor diet will not be lost if you in other ways step down on your regular skin care. This type of skin aging can be prevented and even reversed using topical aplicatons of retin A and Vitamin C, i suspect these will not necessarily be lost.
When you reach your 40's and 50' sun screen and good skin care is as important as ever. Skin are less able to repaire damage from sun and oxidation processes than when you where in your 20's. The aging process of the skin speeds up, and it is more suseptable to any harsh evironment like, beach, sun, dryness etc.
What I expect will be lost fairly quickly when one stops using retin A is the thickening effect it can have on older skin. Skin gets thinner as we grow older and it is caused by hormonal changes and other factors. These type of skin changes caused by internal aging processes will not last after discontintuing use of retin A for example.
In short: good skin care must be kept up much like brushing your teeth. How ever I would not compare results of good skin care to Hollywood standards where results are gained more from plastic surgery, makeup, clothes and photo-shop that any type of skin care.