agree with everyone here that if you are interested then of course you aren't too old. And (I'm biased cos it's my subject...) history is a great choice!
There are a few things to think carefully about though, especially in terms of choice of course.
First, if you need to fit other things around it, talk to the tutors to make sure the schedules will work.
Second, it is very important with history to make the most of talking to your fellow students. So, be realistic about who you want them to be, and think hard about whether to go for a course that is aimed at mature students (OU and Birkbeck spring to mind - and Birkbeck's history dept is GREAT). Teaching mature students is brilliant because they tend to be committed and interesting. Many 19 year olds are also those things. But many are not, and it can be frustrating - e.g. if you are left to carry class discussion, let down by people with whom you are supposed to be doing a presentation, or whatever. On the other hand, they may bring other equally good things to the table e.g. different perspectives from your own, and they have been in school more recently so may be more clued up about essay writing etc...!
Finally, look hard at what you will be studying. If you love medieval history, for example, you need to make sure you can do lots of it!
Good luck - it's a wonderful idea.