In order.
Ashley Williams is the human Soldier, so she's going to be in quite good shape. Bit brash, outspoken and racist towards other aliens. Not really that feminine. Late 20s?
Liara T'soni is more of a geek than anything. To use a Star Wars equivalent, she's the Jedi who uses nothing but Force powers. She's something like 150, but think elven standards
Tali I couldn't find a good picture of, but she's always in that suit because she was born on a sterile ship; that suit is her immune system. Probably the youngest, 16-18?