How old do you have to be to add your old teacher on Facebook?

[bluebellbkk]

There is no rule. If you're old enough to be on Facebook, you're old enough to invite anyone else who is ALSO old enough to be on Facebook.
Remember though: it's up to her to decide to accept or not. Many people decide NOT to accept, for a number of reasons that have nothing to do with you personally. So if she doesn't respond, or responds with 'no', don't be upset. You can always email her, or even - gasp! - write her a card from time to time.