I'm going to offer something more psychological, I hope it's not too unwelcome!
Ok, I always think that 'youth' is something that emanates from within. Course, it's got the physical aspect (unlined skin, taught lines, etc), but sometime in my 30s I recall trying to suss out what really made younger women so obviously visibly younger in photo's than older women.
For me, I think it's the eyes. Not so much the elasticity of skin or absence of lines, but something coming from within the person - iykwim?
A younger woman (or man) really does hold herself differently. Her eyes and facial features a somewhat differently animated than a more mature woman's.
If you look at pics of younger and older women side by side, the younger one often seems more self conscious, open (or innocent/eager) in her facial expressions and these alter the way our face looks. An older woman can often seem more relaxed, content, knowing, etc, in comparison.
Why? I don't know, but I think it's about experience, self awareness, the collective sum of our experiences. We literally hold our entire bodies differently as we get older.
And I don't think this is a bad thing! We really do need to accept this and instead of trying to re-create the face of our youth, it would be great if we could start with the face we have now and work to make it just....gorgeous! So forget the word 'youth' and just concentrate on gorgeous/glowing/chic, or whatever floats your boat.
I don't think much of the water drinking, no smoking stuff holds a massive amount of weight anyway. It is good to those for your health though. But I reckon how we age in terms of skin tone, wrinkles, etc is largely down to genetics and only some lifestyle factors.
For me, it's stress and sleep.
I am 43 next month and over xmas whilst relaxed and sleeping well I looked utterly different: I kind of did double takes in the mirror as my skin, eyes and hair looked so lovely. Now I am working again, sleeping less and worrying about some stuff, my dark circles are back and my skin is dull. My face is noticeably more dragged down and tired looking.
So yeh, my suggestions are just sleep and stress reduction. I don't think much of products, apart from very subtle botox or something, and even then I think even when someone's face looks unlined and tighter, they still don't look 'young'. Even if they look great and more beautiful than any 20 yr old. See, youth isn't the definition of beauty, to me. Feeling young is!
Sorry for length..