The law says what should happen but what does happen is often a very different thing.
The 1851 schools act repeating the 1704 Act required that children be sent as soon as they were capable of receiving instruction and to stay until they could read English distinctively.
This Act was superseded by the 1872 Elementary Education Act which may have laid down a fixed age rather than the capability clause so that is where you should look (I think the statutes are online - if not try Tynwald Library)