Playpen Time began for our little guy when he turned one in March, and so he has begun to spend about 20 minutes a day in the playpen, by himself and with a few toys. Some of our other little ones couldn't jump right into a twenty minute period, so they began with five minutes or so. At least two that I can recall screamed. Loudly.
For the only child, a time in the playpen or fenced-off area can be a great way to learn to entertain him or herself. For the child with a few or many siblings, this time is a valuable lesson in self-entertainment because typically a little guy can come to rely on everyone else to keep him happy.
We must somehow equate freedom of movement with happiness, but the reality is that young children aren't happy without boundaries. A little one who has learned to sit still on mommy's lap for more than a few seconds can be a real joy to take to the doctor's office, on a city bus, or in a worship service.