well i checked for for ya (my dads a secondary school teacher and brings students every year). He said you can go any year you like, theres no age limit, althought usually there is problems getting tickets. Order them early, best chance is through your guidance councilor in school. I remember going myself a good few years ago now, was well worth it :) Although at the time i did the open days at the colleges and those things coz it got me a day off school..
As for doing games i recommend if you want to do coding, then honours leaving cert maths will be a huge advantage. Coz you’ll do alot of calculus (very handy) and vector\matrice\trig\coord geometry. These are the fundamental maths for 2d\3d graphics. Also physics would be very useful. If your good at maths and your school has applied maths, this subject might also be worth doing, definately the most relevant subject for games i reckon.
Then ideally some sort of computer science based course (not necessarily a specialised games dev course coz there rare at the mo, over here anyway). Just a course which has a large content of programming, it doesnt necessarily have to be c++, because typically cs degrees do whatever language is in fashion at the time. Before i started it was c++, then when i was in college it was java and now i hear its c#. Key is if you know one language then you’ll learn new ones pretty quickly. The important thing is that your taught formally how to break down problems and solve then using programming skills. Its a different way of thinking about problems.
Can only speak from my experience but i did a cs degree and then a msc in ai both in u.c.c. I didnt do anything specific in games(i.e. a games degree). Although my msc had some games related ai and now i managed to get myself into the games industry as a games programmer(soon to start, sept). So it is possible to get your foot in the door, without a specific background\education in games.
p.s. if you want to learn coding i would recommend basic or dark basic (for games), they’re easier to learn than C++. C++ is really a language you learn after you can code abit. Lots of info on the web on basic and its pretty powerful. Dark basic can be used to create some very cool games with a small amount of code. Also dark basic also used to offer a school license so you might be able to get yourself an educational copy. If not i think its like 50 dollars or something