Never mind your age, lots of writers have this problem.
We tend to speak the dialect of our surroundings--if you're with friends, you use the same slang, lines from jokes, etc. so that can seem as if everyone sounds the same.
I noticed that when I went to the south--Tennessee and Georgia from Connecticut, I went from a clipped tone to a drawl, saying 'y'all' instead of 'you all', for example. When I went to Scotland I found myelf saying 'Aye' instead of 'Yes'. I didn't do it deliberately, it just seemed natural to speak the way I heard. For the record, anything other than English variants I suck at. (I do speak fluent Cat but I have no idea what I'm saying.)
Do bios for your characters: sex, age, growing up, current family situation, financial status, education, professional training, lifestyles, beliefs, favorite things, etc.
All these would shape a person's dialogue from the rough stevedore dockworker grunts and four letter invectives, to a Wall Street guru who has a passion for Caesar's bio, to a bookstore manager who quotes dead poets, a mother who trains her daughter to the care and feeding of her gryphon, a reluctant princess who slips away to live with thieves, etc.
Our speech changes with our situation: more subservent to those with power over us, casual to compatriots, more authoritative to those serving us. We use technical jargon when explaining work, family slogans and misnomers that the children made up become part of our use, so does frequent saying by friends and lovers, as well as favorite lines from ads, TV, films, theatre, etc.
Only if they're identical twins would two people sound alike, finishing each other's sentences still, one would be dominant.