It is recommended not to use prams for all sleeps (especially for longer sleeps or overnight), partly because the air-flow in many prams is not as good as a cot ot basinette, and partly because it is important for spinal development for a baby to lay on a completely flat surface (although this advice relates more to rockers with a curved surface or prams that don't have a fully flattening lay-back function).

Having said that, our 16 month old still has all of his day sleeps in a pram or carrier, and always has.