Advancement in Scouting is COMPLETELY under the control of the Scout himself.
As fast as he can learn the skills, he can get credit for them towards rank advancement.
TYPICALLY, it will take more than 1 year for him to make it from Scout, to Tenderfoot, to 2nd Class, and then to 1st Class. We do use a "1st Class in the 1st Year" agenda to keep newer boys focused (and soften the "culture shock" from coming from a Cub Pack to a Troop). A Scout can work on the requirements for rank IN ANY ORDER, but his actual progression through the ranks must be sequential.
Some of the rank requirements have SET TIMES that must be met before he can get credit for them and advance. These are clearly outlined in the back pages of his Scout Handbook.
A boy has until his 18th birthday to reach Eagle and/or be a Boy Scout, but the typical age of an Eagle in the BSA is a surprisingly young 14 years of age.
Reaching Eagle is NOT an easy task and requires an abundance of effort and self-motivation. Encouragement at home is CRUCIAL to a boy's success towards his Eagle Rank.