@shyguygamer1,
Never seen the show so ... just went to their IMDb page for some simple math.
Karen Jackson was played by Laura Wiggins from 2011 to 2013. She was 23 years old in 2011. No birth year given on her
Shameless wiki but I suspect it's very close to Mandy given the detailed personal character history.
Mandy Milkovich was played by Emma Greenwall from 2012 to 2016 (starting at the age of 22). The character was apparently born in 1995 or 1996 as stated in the
Shameless wiki. So... she'd been portrayed in her mid teens.