If a whole-time or part-time pensionable officer had attained the age of forty years at the date on which he lost his office, the determining authority in calculating the amount of the compensation payable to him under paragraph 13 of this Schedule may credit him with additional years of service or additional contributions not exceeding one year of service or one year's contributions for each completed year of service between the date on which he attained the age of forty years and the date on which he lost his office, but not in any case exceeding ten years or such number of years as the officer could have served had he continued in his office until attaining normal retiring age, whichever is the less.