Lettuce can be a good stage 1 food for babies but it may not really be a great option it's the baby's first time to try solids. Pureeing instead of mashing can be a great way to prepare them, as lettuce, as all other leafy greens, has choking hazards for babies. We gave our baby the first taste of a lettuce when he was around a year old, since we're sure he's able to bite and chew them properly.
According to Breastfeeding Pinays, you can give your baby a serving of greens starting 6 months, but be careful as these can digest quite longer than fruits.
you can start 6 months onwards. If baby is doing the BLW method it's much easier too. Lettuce is a good start. Cucumber too
1 year old I'd say so they can properly digest the greens and even the corn at uncooked carrots or cucumber.
6months old onwards. Remember to give in small portions. Babies stomach are immature still. Go slow!