1) Consider using an adult duvet print on a big boy bed. Accessories and toys can keep it younger.
2) Pick furniture that can grow with your kids. These shelves and crates are great for storing toys for toddlers and books for teenagers. This is the green route as well - this furniture will last for a lifetime instead of just being outgrown after a few years.
3) You can find prints that work for kids and adults. Here, a beautiful nautical print makes kids and adults happy.
Same sentiment as above, but with a killer oversized map. It's educational and a great graphic.
4) When considering a rug, try to stay away from baby-ish prints. However, feel free to go nuts with bright color.
5) Ugly Dolls are always the perfect modernist accessory for their modern kids.
6) Big Boy Built-Ins are a great place to keep toys and books neat and organized. If this had been my room when I was a kid, by the time I was a teenager those shelves would have held stacks of Benetton sweaters.
7) Think outside the single bed. This four poster is perfect for a princess, and the room can easily transform into a guest room when necessary.
8) Keep the wallpaper sophisticated. This is a wonderful pattern that works for all ages.
9) Similarly, keep the kiddo-stuff casual. Decals like that wonderful giraffe are the perfect way to add young touches that you don't require buckets of Killz a few years down the road!
Ditto for deep space decals.
10) When the playroom extends into the rest of the house, keep everything portable. That is to say, have shelves, trunks, baskets, etc. so that toys can be enjoyed all over the house but easily neatened up and put away.