It depends on the school, but many require Air Law out of the way before 1st solo (not a legal requiement though) so I suggest you speak to your instructor about this.As mcgoo says, 1st Solo is normally one circuit, but some people who fly before their 16th birthday have gone well past the stage they would normally have gone first solo and can end up doing more, but this will only happen if your instructor is happy. This also applies to you actually doing your solo - if your instructor is not happy about the conditions and your ability to cope then he will not send you, even if it is your birthday (treat it as a good airmanship lesson if you do not get to do it). If he sends you then you can feel sure you are ready and you do not need to be worried that you are ready - just enjoy it - good luck.