I shall provide you with an explanation of the usage of the phrasal verb ''break in'' in the context given. On the average, the phrasal verb ''break in'' has the meaning of ''to force entry into something (often a building)'' or ''to interrupt someone or something'', like the examples below:
''The thieves broke in through the back door and stole most of our stuff''.
''I am sorry to break in, but I really need your help right now''.
*** HOWEVER, in the context concerned: ''You've heard hundreds of people of all ages explain their mediocre performance in life something like this: I'm too old (or too young) to (break in) now'', the usage of ''break in'' refers to the beginning of an activity or undertaking - it is used to mean ''start in a certain activity, enterprise, or role (of some kind)''. So, it is as if people tried to explain their mediocre performance in life by stating: ''I am too old (or even too young) to begin a career/have a professional life now...''
Have a look at these other examples below:
E.g.: ''The Senator broke in during the war years''.
E.g.: ''She could break in and have success in life - I mean if she tried to!''
E.g.: ''My father is not that old to break in - he might be as good a cop as you are!''
E.g.: ''When my father left us, Mom decided to break in - she is a great lawyer now!''1
Hope that helps!!! :)