Young Abraham Cupid: In the Bible, Abraham was 100 years old when his son Isaac was born, so Abraham was very old, yet young enough to be a new father. [Abraham could also refer to Adam Bell, a famously accurate 16th century archer.]
Similarly, Cupid was said to be the oldest of gods (because people have been falling in love forever), and the youngest (because he is always depicted as a very young boy).
Mercutio is scoffing at Romeo, who (like every lover) thinks his love is something new and unique. he that shot so trim: "Trim" means adeptly, accurately. In a well-known ballad during Shakespeare's time, "King Cophetua and the Beggar Maid," Cupid is called "the blinded boy that shoots so trim," but Mercutio is still scoffing: in his opinion, Cupid didn't shoot accurately at all when he made a king fall in love with a beggar maid.