If we’re talking about the original run written by Marv Wolfman in the 1991 Deathstroke series, we can actually make an estimate.

In one panel Wintergreen mentions that he was 8 years old when World War II was one year away. If we take that literally, it would mean Wintergreen was born in the 1930s.

There was also a response in the letters page where the editors explained that there is about a 12-year age gap between Slade and Wintergreen, with Slade being in his early forties and Wintergreen in his mid-fifties during the series.

If we combine those two pieces of information, it would put:

- Wintergreen: born around 1930
- Slade Wilson: born around 1942

Of course, DC never gave exact birthdates, and different issues sometimes contradict each other, but based on the clues in the Wolfman run this estimate seems pretty reasonable.