Well, at the end of the series, both Harry and Brandon/Beyond the Grave are in their 30's. Probably around 33-35. Minus 13 years when Harry killed Brandon, they were around 20 then. Than just minus a few years from that point to the start of the series, they'd be around 17 or 18.
Actually, I may have gotten this part wrong now that I think about it. I think they were in the Organization for around ten years before Harry killed Brandon, I think... If so, that would put them in their mid-40's at the end of the series.
As for Mika, she's 13 in the series. It was 13 years between when Harry killed Brandon and then when Mika came into the picture and met up with Brandon. And if I remember right, Maria was pregnant when Brandon was killed.