Astarion wasn't a character I used very much until Act 3, which was when I decided that I wanted to finish his story. So I kitted him out and I actually had some really good gear and I came out with a really- And then I was like, "Wow, Rogue is really fun to play, dang."...
Therefore, it would be sensible to infer that Astarion was at minimum level 11, although he may have been higher at level 12 or 13. Finally, Shadowheart, who was a member of the Sharran Cloister of Sombre Embrace in Baldur's Gate. Looking at the other Sharran that can be encountered...