I agree with the OP. The Combine are basically a metaphor for alien/foreign oppression. Let's take a look at some "real" (film) examples of how citizens may act in the company of Civil Protection.
Also in regards to age, I cannot even say I have anywhere near enough IRL experiences to RP a truly realistic character. I roleplay a 33-year-old woman. I'm a 19-year-old male who lives a generally posh live in a Canadian suburban home with a few friends who pays for his own rent and food. What the hell does that say about me? It means most of Amanda Whitley is ripped from books (50%) and the remainder is ripped from my own personality, others I know and a tiny little bit from television programmes.
If you're less than even the legal age to drink or drive in your country (19 and 16 respectively in Canada), how can you even begin to comprehend what it's like to be an adult? Most of you are younger than 20 years old and have likely never lived on your own, had sex, drank legally in a bar into the wee hours of the morning, done hard drugs, had a full time job, been in a serious relationship, traveled on your own or with your significant other, etc. Most of you have not experienced the major facets of being an adult which is true independence.
But that does not mean you cannot imitate it to the best of your ability. I implore everyone to read mature books, hang out with older people, etc. to get a sense of what it's like to create an adult character. Though none of us will ever be perfect making a realistic character who's older (or in the very rare case) younger than us, unless we're writing an auto-biography.