Firstly, age has absolutely nothing to do with how to pull of a classic style. In many ways, I would argue that the younger man can wear a straight up classical style more easily than a more mature guy (such as myself). Done wrongly, a more mature guy can end up look like an actual geography teacher rather than pulling off geography teacher chic, for example. As a young guy, you have no fear of looking like a geography teacher.
The key to pulling off classical, formal, tailored looks in a modern way is fit and cut. A sharp, slim silhouette brings a classic outfit bang up to date. Wear mis-fitting, baggy items and you just like you're wearing old man's clothes. That means blazers should be in a slim/modern fit and nipped in at the waist by a tailor if necessary (not as expensive as you might think), trousers/jeans should be skinny (without being clingy), and turned up above the ankle (I wear tweed trousers, chinos, indigo denim all like this), shirts shouldn't billow out at the waist.
Another important thing to do is to have one eye on fashion and one aye on style: those cable knit jumpers, fair isle cardigans, nordic patterns and tweeds might well shout 'fashion savvy' now, but next AW they may well be back to shouting 'dressing like an old man' again. So, mix fashionable and classic carefully.
Be bold with colour. Men have only recently begun to really to be comfortable with colour (and many yet still aren't). A pair of red, green or blue chinos, with your tweed blazer and brogues will give a definite modern edge to an outfit. It can be as simple as bold socks if you aren't so brave with colour or you can go the whole way and invest in a boldly coloured coat.
And I've said this before, and it's worth repeating, the haircut makes the biggest difference in looking like a fashion-conscious stylish chap and someone who hasn't a clue.