Longitudinal studies are of this quality are a rare treat these days.
Starting in 1985, this study tracked the health (and in particular) the weight of more than 2700 people over 30 year. You can read the (admittedly rather medical) report in the JAMA Internal Medicine journal here: http://archinte.jamanetwork.com/article.aspx?articleid=2473630
I’ll cut to the most interesting finding.
Those participant that started exercising (on a treadmill) in their 20’s had significantly better health outcomes than those who waited until their 30’s to start… with all other factors taken into account.
The irony here is that in your 20’s you’ll enjoy a great body, lots of energy and – well – exercise can seem an extra which is not needed. Check out the linked study for yourself – it convinced me!