My DP sounds almost identical at that age. He was 27 when we met, and I just 22. It was ok at that age, although I did feel he was getting a bit old to be acting like a 21 year old. He was 30 when our twins were born, and it spiralled out of control.
So much so that after one particularly alcohol-fuelled evening where I felt things had gone beyond ok, I packed the kids in the car, said goodbye and that I didn't know when I would be home. He was miserable. So was I. I am stubborn though, and wouldn't leave unless we had tried everything, which we hadn't. And plus I didn't really have anywhere to go. SO, I came home and told him that he was way out of line. He stayed sober for 11 months.
Yes, he did start drinking again, but never in the same quantities.
In the past, when he was drinking it would trigger a physical reaction in me. I would be tense, walking on egg-shells and feeling sick, never knowing how the alcohol would react with him. (I will stress- he is and never has been violent towards me or the kids- just stupid risk-taking behaviour- burn-outs, public urination etc)
He is NOTHING like that now (although he is now 35), but I still have the same reaction to his drinking. He can and does stop at one or two, and I even got the surprise of my life this morning; he is currently away on a boys weekend in Canberra at the Summernats- usually he gets blind drunk- he has had 3 very quiet nights so far, gone to bed at a reasonable hour and behaved himself very well.
My point is, yes- they can and will change if the need arises for it. It took time- I've been with my DP for over 8 years and in the beginning I remember asking his friends if he was ever sober.. now he rarely drinks and behaves himself well. I never, ever nagged him about his baviour, but I made it known to him how his drinking affected me, how it made me feel and the negative impacts it was having on his life (health, money, social etc).