According to the Berserk Official Guidebook, as of the Fantasia Arc, Rickert is 19 years old, 166 cm (5'5"), and 61 kg (134 lbs).

Is Casca older than Guts?

As such we can deduce that ages of the major characters at said point were: Guts and Casca were at least about 22 years old, Griffith at least about 24-25 years old, Rickert at least about 17 years old, and Princess Charlotte at least about 20½ years old.

How old is Farnese?

Notes. According to the Berserk Official Guidebook, circa the Fantasia Arc, Farnese is estimated to be 19 years old, 162 cm (5 ft, 4 in), and 48 kg (106 lb).