My youngest had it from the age of three months. It eased a little after I started taking him swimming, strangely enough, and eased even more when I stopped giving him formula milk and let him drink cows' milk.
Aloe Vera cream was recommended, but it actually made the condition far worse.
The doctor also advised me to allow - even encourage - my son to pee in the bath. Clean urine is, apparently, good for the skin (hence rookey soldiers were advised to pee on their feet in order to toughen them up for marching).
There is a cream on the market, Eucerin, that we found especially effective. One of its active ingredients is bovine urea (in other words, cow pee). You can get it from boots.