I’d be surprised if he is 43. If so then he certainly aged better than I did at that age!!!
This is from the PGMOL press release ...
Harrington (below) spent four years as an EFL assistant referee and four years as a National League referee before promotion to the EFL in 2012. He has been a Select Group 2 referee since 2016 and has refereed nearly 200 EFL matches.
Even if his 4 years as an EFL assistant were concurrent with his 4 years as a National League referee that would mean he was promoted to those roles in 2008. So if he really is 37 that would mean he was a National League referee and EFL assistant by the age of 24 / 25. Not impossible but very would be very quick progress. Although he has a linked in profile that says he finished 6th form college in 2002, and that indeed would make him 37.
I seem to remember watching him referee an Under 18 game at Newcastle against Leeds 15 years ago, which would seem to fit in with that timeline.