Kids love to collect things. With a little luck and a lot of effort, children might be able to build up valuable collections.

There is no set age when collectibles become assets, but most are at least 30 years old before their worth increases. Some collectibles known as "investment-grade" can become very valuable if they are in perfect condition and in demand.

A market is created for a particular collectible item each time a purchase or sale is made, and the price is determined by how much a buyer is willing to pay and how much a seller is willing to accept.

The good news is that children could discover a great deal at a great price. The bad news is that with every purchase, they may be buying treasure or trash. If only it were easy to tell what will be valuable. Unfortunately, there is no formula. Perhaps it's better just to encourage your children to collect things that will in the future illustrate their own childhood.

The first rule is to have fun. Collecting can be time-consuming and expensive, so while old Barbie dolls may be valuable, if your children think Barbie is dorky, they'll never be passionate about the collection.

Baseball cards were first introduced in the mid-1800s. The price of a card set is determined by a few factors: grade, age (older cards tend to be worth more), popularity of the player and rarity. Complete sets can appreciate faster than a single card. It's estimated that over the years, Topps sets have appreciated in value between five and 10 percent a year.

Some other possible items are:

Lunch boxes. That "Jetsons" lunch box you could have bought for $1 at a garage sale in 1980 could be worth $200 today.

Movie-star memorabilia. Look for posters, dolls or buttons.

Maps. As borders of countries change, old maps can be future collectibles.

TVs. Think twice before you throw away those old black-and-white sets in the basement.

Collectibles can be found anywhere. Have children start in grandma's attic; among old tools, books, albums.

Go to flea markets and garage sales together. Also, antique shops. Young collectors can get to know the dealer and ask to be notified if something special comes in.