If you find yourself worrying about the age gap then they are too old. When love is conquering an age gap, neither of you give it much thought. Date whom you adore and enjoy it without worrying about age.
The rule of thumb is to take your age, cut it in half and add 7. That is the lower limit of dating. So, if you are 57... 57/2 + 7 = 35.5. That is considered the lowest socially acceptable age before people raise eyebrows.