I hear it all the time. "My child is 15 months old and he climbed out of his crib! I can't believe it! Is it time to switch to a toddler bed?" With all of my heart I respond, "DON'T SWITCH YET!" That is the biggest mistake parents of toddlers make (next to switching to a toddler bed for baby number two) in my opinion. He is to little to handle that freedom. Sleep time whether naps or night time will be unpredictable and need lots of extra time and attention.
My son did the same thing at 15 months. We did a few things to resolve this bump in the road:
1) We bought a baby monitor with a video cam on it so we could see him in action. If we saw him lifting his leg... we would immediately go in and firmly say "NO CLIMBING." This happened repeatedly.
2) We applied a swift tap to the thigh each time we were able to catch his leg up there. More often than not daddy's deep "No Climbing" command did the trick. It didn't take too long after that.
3) We lowered his crib mattress - it was already as low as it would go so ingenious hubby took the drawer out of the bottom of the crib and dropped the mattress to the floor. The rail was so high it almost went above his head while standing.
It is very common for toddlers around 15-17 months to try climbing out of their beds. I hear it over and over. Some would argue is that the crib is no longer a safe place for the child... but having full reign of the house at night while you sleep is a recipe for disaster! Plus how will you keep him in the bed when it comes time for sleep? Before he transitions from crib to big bed you need to work on him willfully obeying you. It is possible. You have to mean what you say and follow through.
If you say, no climbing and he climbs you need to give him a consequence. It may only need to happen once or twice but it needs to be enough for him to not want to do it again. It is a tough step in parenting but these are the times that they test you to see who really is the boss.
Be wise, be foreword thinking in your parenting, and have a stronger will than your child.
By the way, my son is now almost three and is in the 97th percentile (tall) and he is still in the crib. We will switch soon but at this point he likes the security and we like knowing where he is when he is told when to go to sleep. I have friends that did the switch at 16mos and they never know exactly how long the child has slept because he gets up in the night to play and falls asleep in the floor. This child has also stopped napping because there is nothing keeping him in the bed. That make for long cranky days!
If you are considering doing the switch because baby number two is on it's way. Think about it. Baby number two has been up all night and you have been too. Your sweet little toddler has gotten a full night of rest and now it is a new day. One o'clock comes and you covet this time of peace. Nap time! I mean, you really are looking foreward to the break. You pray they both sleep at the same time. A mom who has just given birth (horemones raging), has been giving her chest for nourishment, and hardly has time to shave both legs on one day let alone even get a shower, has had to keep toddler from holding up his prizes new sibling for show...etc. YOU WILL GET THAT BREAK!
Now little toddler may need you to lay down with him to get him to sleep, he drops like a hat off to sleep! You tip toe out of his room, you drop your heavy body on the couch for some much needed shut-eye. And you are off. Literally 5 minutes later you see a happy little toddler stareing you straight in the eye! And the battle is on. Take the little guy back to his bed and start it all again. By this time, your new bundle needs to eat...... and mom has not had a break.
This is the real deal. It happens. These are the things that moms turn ANGRY about and begin reacting and hurting their children. Moms need their sleep.
All this to say. Keep your toddler in his bed as long and you can. It can only help.