As an addendum to the question I know that many high ranking officers who had served in World War One also went on to serve in World War Two, but what about enlisted men?
I apologise if this is a frequently asked question, I'm just curious as I recently discovered my great grandad served in both.
You would have needed to be young enough to enroll when WW2 erupted, and old enough to also have enrolled in WW1 before it was over.
The specifics of drafting citizens varied from country to country. As an example, French military service was 3 years long starting at age 20 after the 1913 "Three year law" got passed to compensate for its lack of population compared to Germany. Its length dropped to a year after WW1, and went back up to 2 years by the start of WW2. You could could get drafted up to age 45. In Germany, Volkssturm drafted all males aged 16 to 60 who were not already serving in some military unit towards the end of WW2.
As to volunteers, I've no idea what the youngest or oldest acceptable ages were for the French army and how prevalent youngsters and oldsters were. But there certainly are examples of both. In WW1 for instance, Sidney Lewis was a UK soldier who died on the battlefield at age 12; Momčilo Gavrić was a Serb who entered his unit at age 7, got promoted to corporal - and given an actual uniform - at age 8, and survived both wars; and Henry Webber was a UK soldier who died on the battlefield at age 67.
Anyway, this is to say that most in their late-30s to early-40s in 1939 potentially did both wars owing to having been young and old enough to get drafted in both. And then you need to account for volunteers on both ends of the age spectrum, and Volkssturm in Germany in the last months of WW2. I've honestly no idea what the precise percentage of WW2 soldiers that participated in WW1 is, but it was most certainly a two digit number for key European participants.