OP, i'm 25 and I kinda have the opposite experience. At 18 was very muscly (although still babyface like you) and with a full set of chest hair and tattoos on my arms, I looked like 26 at 100kg. Now i'm 25, about 75kg (at least quite ripped) wear nicer clothes etc and if not for the clothes I swear to god I'd look 19-20.

Trick is definitely wearing nicer clothes. And I don't mean wearing your dad's button ups with dress pants everywhere. Just learn to dress yourself nicely, have some staples in your wardrobe such as denim jackets and khaki/olive/black pants/jeans/chinos, white/black/grey shirts, and most importantly... get clothes that FIT you well. Try them on before buying!

The MOST IMPORTANT thing about dressing yourself and the MAIN difference between looking like a scrub wearing his Dad's funeral/wedding clothes and channeling your inner Hemsworth - is having clothes that fit you well.

If necessary, take your nicer clothes to a tailor after you've bought them and have them perfectly fit to you. I'm kinda tall, so I usually have to have sleeves made a bit longer, and when I buy tall ranges, usually make them shorter and widen shoulders. It's cheap don't worry!