Cub Scouts is Grades 1 thru 5 (7 to 10 years of age). In order to be a Tiger in Cub Scouts, you need to be 7 or First Grade...at least that is how they do it around here.
http://www.scouting.org/nav/enter.jsp?s=cy
Tiger Cub Scouts First-grade boys join a Tiger Cub den, where each boy works with an adult partner on the requirements to earn his Tiger Cub badge.
Wolf Cub Scouts Second-grade boys graduate into a Wolf den. They go to weekly den meetings on their own, but their families still help them work on the requirements for the Wolf badge.
Bear Cub Scouts Boys in the third grade are members of a Bear den. They also work with their families to do the requirements for the Bear badge, but boys this old have enough knowledge and skill to take on more of the work by themselves.
Webelos Scouts Boys in the fourth and fifth grades become Webelos Scouts. Webelos Scouts do more advanced activities to get ready to graduate into Boy Scouting.
My older son did scouts for a few years and he enjoyed it. I was really involved with the pack. How fun or sucessful a pack is, really depends on the parent volunteers. Around here, a lot of kids join their pack at their school, if there is one, but some folks join other packs, just because it's a better fit for them.
You can use the link to click on 'Join Cub Scouting' to pull up a form to type in your zip code to find packs near you.