Yes, Jarra is eighteen, and I agree with you that it does seem to have happened quickly. I keep a time line though, as an aid with fitting the different events in sensibly, and I can assure you that that's where he is now. This chapter itself covered more than a year.
Yirgella is certainly impressive. I'd say better in some ways but not in others. I think the extraordinary interest all the AIs show towards Mirri could be recognition that his nature might be something they can never experience for themselves or understand properly. (I forgot to mention in the story that Yirgella made a point of introducing Mirri to each of the other AIs as their 'friend').
Part of his benificence might be a survival mechanism too - I remember at one stage he said that if he did even one thing wrong it would change the way humans would regard AIs forever.