In Raphael's House of Hope, before the portal
Another interesting moment with Luskan portal.
Astarion: Luskan was once a beautiful city. Well... it was a den of pirates and vagabonds, but I don't mind.
Events bg3 1492
Luskan before Spellplague (1385 DR–1395 DR) was also a questionable city (as well as any in Faerun). There was also piracy led from the shadows. And after the plague it turned into absolute anarchy. Maybe he heard stories, or maybe he was making a profit there with the pirates, transporting something illegally is for them. But that's a theory.
Don't forget Astarion's age was changed, 350 to 263.
Idle champions of the forgotten realms says Astarion is 263 years old he's an elf, though he looks 39.
SH is 40 years old and she looks younger, and she's a half elf who's aging faster.
There is no way he was a Magistrate at 39 and looked that way for his elven years. The dates on the gravestone (born 1229 DR) aren't correct.