"When I was twenty, I started working as a security officer."
Translation:Mając dwadzieścia lat zaczęłam pracować jako ochroniarz.
I don't know if there even is a feminine equivalent of "ochroniarz". If, then it probably would be "ochroniarka", but googling it doesn't really give any believable results suggesting that this really is in use. So I guess a woman would probably call herself "ochroniarz" anyway... and it is accepted here with feminine "zaczęłam" as well.
"jak" is "how".
"jaki" is the 'adjectival what', which is easier explained as "what kind of".
"Może ktoś wyjaśnić (explain one time) jak i kiedy używać" (how and when to use - in English this 'when do you use' may be understood as impersonal and equivalent to 'when does one use'; in Polish 'ty używasz' definitely is directed to some specific person).
"mając dwadzieścia", without "lat"? I cannot imagine where it could be accepted... you can omit "years" in English, but not in Polish.
The only situation in which this would be possible is when the context makes it superclear twenty of 'what' you're referring to, but even then it's weird.