This reddit posts suggest that Geralt's age is about 100 years.
The age of Vasemir must be at least 150+ (considering above info) because in the first episode of Hexer, Vasemir is shown old(must be 50+), and Geralt seems to be around 13 years. Now considering timeline of Witcher 3, Geralt seems old, so Vasemir's age must be around 50 years more than Witcher.
The reason for considering vasemir's age is that they are mutants and their ageing rate may be changed, increased or decreased. I can't say anything about that.
However in Witcher 3 Vasemir dies at about 150 years of age. So, the maximum age is about 150 (it can be still more if Geralt crosses 150).
Considering Leo's case (minimum), he seems to be at about 20 years old at the time of his death.
So my question is: What is average lifespan of a witcher?
Also, please give a good source from where the answer is researched and found out.