I used to struggle with this question too. When I asked around in mom groups, the answers ranged from "switched at one year" to "still wearing them at two." Eventually, I discovered that it mainly depends on when potty training begins.
How Long Can Baby Boys Wear Onesies?
When it's time to transition from onesies to separates
When you first hold your newborn, you'll notice their closet is packed with all kinds of onesies. This seemingly simple garment carries nearly all of an infant's wardrobe needs through their first year of life. But as your baby grows day by day, you start to wonder: just how long should they keep wearing these?
Most babies transition to separates between 12–24 months.
But this is just a range. I've seen moms switch right after their baby started walking at 10 months, and I have friends whose kids still wore footed styles to sleep close to age two.
Onesies: A First-Year Essential
Choosing the Right Sizes
Growing Into Separates
The Size Limitation Reality
Here's a practical issue: most brands only make onesies up to size 18–24 months. After that, even if you want to buy them, they're hard to find.
Walking Changes Everything
Learning to walk is also a turning point. Once babies start walking, they're so excited about this new skill that they refuse to lie down for diaper changes.
The Transition Timeline
0–12M
Peak onesie era for daily wear
12–18M
Walking begins, changes become harder
18–24M
Transition to separates starts
2–3Y
Potty training brings full switch
Why Potty Training Is the Real Game-Changer
But I think the most crucial factor is potty training. In the U.S., most kids start training between ages two and three, with boys generally starting a few months later than girls—many boys don't begin until age three.
Why does potty training signal the end of the onesie era? Think about it: during early training, "quick pants down" is key.
Unsnapping the bottom of a onesie and pulling it aside is much harder than just pulling down pants. If there's an accident, changing one pair of pants is a lot easier than dealing with a whole onesie. And when children are learning to use the toilet independently, you want them to be able to pull their pants down themselves—this also prepares them for preschool.
So for baby boys, since potty training tends to start later anyway, they naturally wear onesies a few months longer.
The Daycare Factor
If your child attends daycare, many centers directly recommend that parents dress their kids in separates—it saves caregivers the hassle.
Why Onesies Have Stayed Popular for So Long
Onesies have remained popular for good reason. They stay snug against the torso so the tummy never gets exposed—they work as a base layer in winter or as a standalone outfit in summer. For diaper changes, you just unsnap the bottom, and with newborns going through about ten changes a day, this design is essential. Oh, and that envelope neckline? It's a lifesaver during "blowouts"—you can pull the dirty clothes downward instead of over the face. Anyone who's been through that knows exactly what I mean.
A Note for Boy Parents
Speaking of baby boys, any parent of a boy has experienced the "fountain surprise" during diaper changes. Some onesies are specifically designed for boys with better coverage—something worth looking for.
When to Transition
Potty Training Begins
Once potty training starts, it's pretty much time to switch.
Fit Issues
If the onesie starts feeling tight, riding up, the neckline is getting snug, or changing becomes a battle—those are signals too.
The Sleep Exception
Nighttime is an exception though. My kid switched to separates during the day early on, but still wore footed sleepers at night.
Safety First
Since infants can't use blankets, footed sleepers plus sleep sacks are the safe way to keep them warm. Many brands make footed sleepers up to 3T or even 4T—we kept using them until almost age three, when he said he felt too hot.
Shopping Tips to Avoid Common Pitfalls
One tip when buying: don't stock up on too many newborn sizes—many babies outgrow them within weeks. Basic styles from affordable brands work great—Gerber, Carter's are both solid options. They grow so fast that many outfits only get worn a few times anyway. If you're buying a gift for someone else, skip the newborn size—go for 6 months or larger, which is much more practical.
The Bottom Line
So to sum up: typically between 12–24 months, and it's pretty much time to switch once potty training starts. Because boys tend to begin potty training later, they might wear onesies a few months longer than girls.