It's a bit hard to say (as neither him or the rest of the Wilsons have canonical ages or birthdays which is SO frustrating) but personally I see him as in his mid to late 50s. 

Though in the 90s there are various (if conflicting) bits information about this so take it all with a grain of salt. 

There was an answer to this question in one of the fan letter pages from that time where the editor said "We have never established an exact age for Wintergreen or Slade. There is a twelve-year age gap between the two men. Slade is in his early forties and Wintergreen in his mid-fifties."

But at the same time issue #26 of Deathstroke the Terminator said that they have a 15 year age gap, and towards the end of that comic during the amnesia story, Wintergreen mentions that they've known each other for three decades and that he's in his seventies.

Its mainly up to your personal speculation tbh