Student or Learner
1. How old do you think I am? (It is OK)
2. How old do you think that I am?
Do you think that 'that' in the second sentece is redundant or is it OK?
It's redundant and sounds unnatural. However, it's interesting to note that in the reply, "that" would sound fine. "I think that you're about 25". It's fine without "that" of course, but it's also OK with it.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.