The first season of The Witcher left several unknowns to those who have only seen the series. Readers of the novels and players of the CD Projekt saga are not so lost with the universe in which the story takes place.
Now series showrunner Lauren Hissrich has spoken in a recent interview that Geralt is almost 100 years old at the beginning of the series. At the end of the season, he would be almost 150 years old. As you may already know, this is due to the mutation that causes their aging to be slower than that of normal humans.