The only problem with exercise is that the two cords at the front of the neck will eventually stick out...and that look isn't so nice to me. Of course, if one has gained a lot of weight, then lost it..or maintained very low body fat...that will encourage a strange neck appearance once the age that collagen in the skin starts to decline and loose its hold..
Allergies or irritants in some products...maybe fragrances and high AHAs...could encourage drying of the neck/chest skin over time...but young skin will still have its nice plump support...not to be noticed until later years.
Tanning/burning that area will eventually cause the skin to lose its elasticity much earlier than normal.
I think having the neck "flexible" and stretched helps, no smoking, good moisturising products, being generally well hydrated and having a well balanced diet are all continually helpful. Good genes helps too!.
Basically....sun is the #1 culprit. All the areas on my fair skin that burned when I was a kid are definitely starting to show now..my eyelids being one. I haven't been in the sun since the early '80s when I worked cosmetics and saw 40 year olds looking like they were 60 year old leather. ugh. But, I still have the melanin spots from then....
Being 54 has been humbling...although I don't look it...my eyesight has me using magnifying mirrors...yep, humbling.
My half-sister, who has sunned all her life has neck and arm skin that's sagging badly but she grew up in hot/sunny climates. She still looks good for her 59 years, but the neck and arm skin detracts from the rest. She also smoked for about 30 of those years.
h2u