Dear Eoin,

Your age would not be a problem. Physical age, however, is only one element of several that point to a vocation.

Firstly, your interest must be personal and free (no-one forcing you) for a spiritual motive, and that would seem to apply in your case (you know a vocation comes from God, you want it, you can't wait to start...).

Another important element that tells if there is a vocation is a man's maturity. People sometimes confuse age and maturity and that is a mistake. I know men of 50 who are irresponsible and can't control their passions, and young men of 18 and younger who understand what a commitment is and can keep their word. You tell me which is maturity.

The final element is health, both physical and psychological. 

So, in theory 18 is not too young as long as the other elements are present. 

Now, there are some seminaries that will simply not accept candidates who have not done a degree. Others do. You will have to search. But as you do there is an important question you should ask Christ every time you pray and especially after receiving him in Communion, does he want you to serve him in your home diocese or in a religious order? The answer will tell you if you should pursue this with your diocesan vocation director or the one from the order you are interested in. God bless.