There is nothing wrong with "Then I was two years old," but it seems like this translates to "at that time I was two years old."
I agree. Pada waktu itu = at that time. I base this on knowing that 'pada waktu' is the phrased used to say, At the time ...'
Oh yeah true it can be "At the time..." as well as "At that time..." another case of both "the" and "that" working as "itu" translations