That's a very good question and there are many different options. It really depends on your budget. Here's why. Almost all of the basketball systems adjust up and down-down to a height of 7.5 feet. There are many five-year-olds that can play with a mini basketball and have a lot of fun. If you're 5 1/2-year-old is average size a mini basketball may be required at 7 1/2 feet rim height. Otherwise, you might consider one of the youth portable goals. They would be good for a few years and would allow your child to play at an even lower height. Model number 90022 specifically goes down to 5.5 feet and up to 7.5 feet. It doesn't fit a regular size ball so it would be required to play with the Junior size or a mini size. Now, if your boy is a tall 5 1/2-year-old you could go with any of the standard size basketball systems including this one model 1268 which goes down to 7.5 feet and up to the standard NBA height of 10 feet. In fact, that's a very good entry-level basketball system. If you find that he really takes the basketball and you want to invest in a more expensive system there are plenty of systems that range from several hundred ($300 is about the average price range) up to several thousand dollars. If you have any other questions please let us know. We are happy to help.