Most Helpful Girl
If you physically look 25 (or in your 20's) and act like it, it will show in whatever outfit you are wearing. Clothes say more about who you are than how old you are.
I don't know what your wardrobe consisted of before but most 25 year old guys I know stick to the t-shirt and fitted jeans with belt combo.
When they spice it up, it will usually be with a plaid shirt, a jean jacket or a leather jacket.
Sweaters and jeans are also a great casual combination.
A pea coat, I think, is very mature and great wardrobe piece for guys.
Accessory wise: beanies and belts =)