Some other npcs can sell you more clothes, Billy for exaple sells peasant clothing. There are also some quests / events that will also give you new clothing. Another way is to survive for one year, go to the beach and "finish" the game. This will unlock new clothing depending on the ending you got. These clothes can be put on in the new cave you unlock and they change the look of anything you are wearing. "Transmog" I think is the term other games use for this.
As for aging, yes, the only way is to play the game and wait. Your looks change every year automaticaly. Also, finishing the game will save the age you reached and in future playthroughs, you can switch your characters look to any age between 10 and your max reached age.
Both the unlocked clothing and age switching is cosmetic only. It won't raise your stats and people won't react any differently to you.