My husband and I married young. A year later, we had our first child. We were (obviously) still young. And so were our friends. Only these friends were not parents, and most of them were not even married. They kept odd, college-like hours. And we had two choices: we could ditch our friends for the baby, or we could work out a compromise.
The playpen is our compromise. It serves as a portable nursery for Baby. We are now older and wiser, and yet we still make the playpen central to child-training in our home.
There are many reasons to train a baby to sleep in a playpen. The most obvious is that when parents travel, they usually bring along a playpen for the baby to sleep in (or the hotel provides one). If the baby has not been trained to sleep in the playpen, he may not sleep at all, turning a vacation into a nightmare. But this benefited us a great deal even at home, especially when we had only one child, because we could go to a gathering of friends and stay as late as we wished. We simply brought along the playpen and set up a makeshift nursery in a back room. Our son and daughter both used a playpen in this manner until they were two-and-a-half. Our younger daughter still uses one. It is quite freeing to not be bound to the house simply because it is naptime or bedtime.
From the very beginning, a baby can be trained to sleep in a playpen. This works best when the child is not also learning to play in the playpen. Playing and sleeping in the same place might get confusing. Because we did not need our children to play in the playpen (we usually confined them to jumper or saucer if they really needed to be kept in one place), it was easy for us to focus on the one goal of sleeping in the playpen.
The question is usually how to do this. What has worked for us is to start young--about the time the baby has outgrown the bassinet and moved to a crib. Our older daughter was more difficult to train than than our son, but all three of our children were eventually trained.
Regular practice is key. Think back to the baby that ruins the vacation because they are standing up, screaming in the playpen. Most of the kids that do this have never or rarely slept anywhere other than their cribs. The playpen was a strange scary place to that child, and so he had trouble relaxing and going to sleep.
How much practice is needed will vary by child. Our more difficult child needed lots of practice for many months. I simply had her take one nap a day in the playpen until she was no longer resisting (resisting, by the way, was not screaming, but just a tendency to fuss a bit and take longer to fall asleep). Then I lowered the frequency to about twice a week. Once she was over one year, she outgrew the need to practice, and slept in the playpen whenever we required it of her.
One last thing that helps is to use sleep props. We put down the same fitted sheet for her to sleep on every time, covered her with the same blanket, and let her cuddle with the same bear. We make sure she does not have access to these items unless it is time for sleeping, and so the items serve as cues, communicating to her that it is time to settle down and sleep, regardless of where she is.
Training to the playpen is very rewarding. It gives a family portability while also serving as a comfort to the baby. Everybody's happy and, most importantly, Baby stays well rested.