How to understand "going on sixty" below, plz help me.
I was thirty-one going on sixty, living dog years—aging seven years for every year.

Question

Answer

This means that the speaker is feeling older than is natural for their age, which is stated as 31.  Because dogs have shorter lives they are considered to age 7 times as fast as humans based on average lifespans.  This gives the phrase 'dog years'.  For example, my dog Fresno in my profile picture is 5 years old but in dog years that is equivalent to 35 in human years.