For the probate you found, the GRO death index gives the date of birth as 17 Jan 1921, which, if this is her, would have made her 20 when sentenced. However, people who register deaths sometimes don't have completely accurate information, so if, for example, you have her date of birth as 17 Jan 1922 there's a good chance that this is the right one.
Note that this is from the version of the GRO index that you can see at Ancestry - the GRO's own site (which can be used to order a certificate) only gives the year of birth.