Basically the issue with the knees is you don't want to start riding a horse too strenuously until the knees close. For each breed it is different. Some close at 3, others, like drafts and WBs, can wait until 5 or so.
the act of making or enacting laws. 2. a law or a body of laws enacted. Origin of legislation.
A Bill is a proposal for a new law, or a proposal to change an existing law, presented for debate before Parliament. A Bill can start in the Commons or the Lords and must be approved in the same form by both Houses before becoming an Act (law).