I have a rule of thumb, if you are capable of getting it out, playing with it, and moving to a different location, you are capable of putting it back. All of my boys started picking up after themselves pretty much as soon as they could walk. Mostly, because with three boys the playroom can quickly become a nightmare of kid clutter. Here's how I taught them to clean up.
1. Babies love making things disappear and reappear. make clean up time a game of making the toys disappear. "Where is the toys hiding place?" ...
2. Babies don't need to be overwhelmed with toys. Pick about 10 items, place them in a basket, bucket, some sort of container. Keep the container centrally located for easy access.
3. Toddlers are notorious for emptying out a toy cupboard, and up and walking away. This is the time that I introduce my rule of thumb. It is also helpful to teach when you are done playing with something put it back.
4. Luckily, I have a son that is particular about his toys. He really appreciates having an organized play space. For his room I use IKEA kids storage cabinets and bins. Each grouping of toys or type of toys has it's only bin. That way it's super easy to get out, and clean up. I remind him that in order to be able to find his toys easily, he needs to put them away where they belong. He's 7 so he gets it.
5. Make it a part of your daily routine. When I am at work, I try and have the kids clean up after about 20 minute blocks of time. We call it a quick tidy. It's just to keep their play space manageable. I simple have them put 5 things that they aren't playing with away. If your kids ever watch The Big Comfy Couch, this is known as the 10 second tidy. Once they do it they go back to playing.
6. Kids do the cleaning, Moms supervise. I don't pick up! I help shuffle kids about, and help them stay focused. They can do it, trust me. They may not want to, but that's where turning into a game and making it fun helps. Make a big deal about the fact that the room or space looks so good, they appreciate the praise.
7. Use a timer. When I find the boys are lacking focus I say "I'm setting the timer." This means that if anything is left on the floor when the timer goes off it is mine to keep. ( For a while). This is just a little extra motivation to keep them focused. To date, I have never had to keep anything.
8. Make sure they ask their friends who come over to play to help do the clean up. It just reinforces the behavior for your kids. Also, babysitters/caregivers need to be on the same page.
Bottle line, If they get in the habit, and you positively reinforce it, they will clean up no matter how old they are.