Becky was around 4-5 months I think. She actually would have needed it a bit earlier but I didn't know about night nappies then so I just changed her at every feed (3-4 times a night). And you're right, it's more the stage than the age
Becky stopped pooing at night around that age too so it makes the whole night tasks a lot easier. She still needed a few feeds at night but the nappies lasted us 'til the morning.