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.
The term 'law' is general and refers to the set of regulations or rules to be adhered to. An act is a subset of law. Initially the proposal for a new legislation is introduced in the parliament. At this stage, it is called as a bill. Only when the bill is passed by the parliament or legislature, it becomes an act and comes into force.