With Mama Sanch, I'm always 18 months old. It's an age which works for both of us, and one which I always feel comfortable and natural in. However, it's not a true representation of that age. It just adds a sense of realism to the process of cobbling together behaviours from different ages. At '18 months', I tend to be more verbally advanced than the average young toddler, yet enjoy Infant-style toys which most toddlers would have lost interest in.
When regressing alone, I can go a bit either way, in terms of the little ages I enjoy. Sometimes I like to act like an infant of, say, 6 months old - and sometimes I ageplay as an older toddler, around 2 1/2 years old. I find 'ages' to be a good way to make ageplay feel authentic for us, but I don't get too hung up on accuracy anymore.