A 16 year old girl dating a 23 year old man

Recently found out that my Mom is allowing  my little sister, who is only 16 years old, to date a 23 year old man. I am furious & ive voiced my concerns, but my mom doesn't seem to see an issue with the age gap. It's not so much the 7 year age difference, but the fact that she's only 16!! & he's 23!! What could they possibly have in common? What is he after? & why can't he find it w/ someone his own age? Am i overreacting? Would any of you have a problem with this situation? 
**Update** 
Thank you ladies for your comments. Instead of replying to each individually I thought I would update this post & give a little background into this guy & why I believe he's just no good.
First off he's a cheater. I know this because I know his ex gfs & because we live in the small town & people talk. I'm willing to put my life on the line & say he's already cheated on her, but she won't listen to me because well he denies it. Second, there has been flirting & communication w/ other girls. I know this because I've heard her crying to my Mom about it. They've only been dating for 3 months tops & already talking marriage. It's so odd to me. Do I like the guy as a person? Yes, he's very charming, but I KNOW he will break her heart or worse knock her up & split w/ the next best thing. 
Once again, thank you ladies so much! I hope for her sake it works, but I'm so afraid I'll end up being right.