Any permanent fireman who accepts the provisions hereof may retire from active service at the age of 55 provided he has served as a permanent fireman for a period of 20 years. Any permanent fireman accepting the provisions hereof and having served for 20 years, who shall be dismissed from service after having reached the age of 55 shall be entitled to the benefits of this chapter. Upon retirement a permanent fireman shall no longer be obligated to pay assessments to the retirement fund. The probationary periods of permanent firemen shall be counted as part of their term of service.