MY BOYFRIENDS EX WAS A 14 YR OLD!!1

My boyfriend started dating his ex when she was 14,he was 18,an it bothers me ssooo much that he dated someone that young, i look at her a s achild he was dating a child she wasnt even fully developed into womanhood yet,lik cmon... i aksed him what he seen in he ran he said idk i liked her she seemed mature!!!! What should i think!? and they had been together off n on for 2 -3 years!!!