I can't get a really good look at the leaves but I'd say Acer negundo (Box Elder). It is a type of maple. Very hardy and tends to be a bit weedy in many places with seedlings coming up in the garden and nearby roadsides and vacant land.
They respond well to bonsai treatment but the compound leaves tend to be a bit big for good bonsai. Also has longer internodes so does not ramify as well as other maples.