There's no set month babies "graduate" from onesies. Most parents notice the shift somewhere in the 3 to 6 month range, right when a baby starts rolling over or kicking hard enough that reaching under them for a diaper change turns into a wrestling match. A two-piece top and bottom set solves that: you swap the bottoms without pulling anything over the head. Since our sizes run from newborn up to 4 years, there's no rush either way. Plenty of families keep onesies in daily rotation well into toddlerhood, especially as a base layer under a two-piece set.
Climate plays a real part in the decision too. In hot, humid weather, a two-piece muslin cotton set can feel cooler than a snug onesie because the loose weave lets air move around the middle, not just the arms and legs. If you're weighing sleeves against bare arms for a summer baby, our answer on whether babies need long sleeves in hot climates walks through that. For bedtime specifically, some parents keep a onesie for nighttime and compare muslin pajamas against a two-piece muslin cotton set before deciding what goes in the nightstand drawer.
Once you do switch, wash new pieces the same way you'd wash the onesies: machine wash cold on a gentle cycle, then tumble dry low or hang, and skip fabric softener since it works against the breathable weave. Muslin cotton softens more with every wash, so pieces mellow out fast even after a few wears. If you're stocking a diaper bag or a suitcase, our guide on how many outfits to pack per day for a baby is a good starting point, since two-piece sets and onesies pack differently. We ship worldwide with tracked delivery, and unworn pieces can go back within 30 days if a size guess doesn't land.