Dave who married at 25 and now has a 25-year-old son. How old is Dave now?
You can’t know his exact age from that info — but you can give the minimum.
If Dave married at 25, the earliest he could have had a child is at 25. If his son is now 25, the minimum Dave’s age is
25 (age when he married/earliest when son could be born) + 25 (son’s current age) = 50.
So Dave is at least 50 years old — he could be older if he had his son after age 25 (e.g. had him at 30 → now 55).