The second year Webelos program is designed to end at the end of February when the boys should graduate into a Boy Scout troop. (Link) This permits the boys to get integrated into their new troop and to get used to the Boy Scout routine before the start of summer.
There are many differences between Cub Scouts and Boy Scouts. Transitioning your fifth graders into Boy Scouts while the school year is still ongoing improves the likelihood of a good transition.
If a fifth grader does not join a Boy Scout troop until the end of the school year, his chances of a staying active in scouting are diminished. Many boys and adults will be out of town various weeks during the summer and most troops will be about to head to summer camp. The boy who has just joined a troop when summer starts has not yet had an opportunity to experience the regular routine of camping and troop meetings that occurs during the school year. This makes adapting to the new world of Boy Scouts more difficult.
Since our Pack has begun graduating our fifth graders into Boy Scouts a few months before summer, we have had a much higher percentage of our boys staying active in scouting.
I strongly urge you to graduate your fifth grade boys into Boy Scouts a few months prior to summer.