A demon’s body (as well as Angels) aren’t always what they “want” it to look like. They can often alter their forms, yes! But their default forms are more what they feel like.
Let’s look at Brian the Organ Duck. He’s… a duck made of organs. Which makes sense, because he tends to deal in trades involving organs. That’s his preferred “taste” in power, which most demons have. Now, the duck part? Who knows. Maybe he has a very particular memory concerning ducks. Maybe ducks are another lesser-known taste of his. Maybe he just think ducks are neat. Or scary?
A demon looks the way it looks due to a balance of their interests,
self identity, their past experiences, and even significant power sources that make up a decent part of their being.
With Dipper, he’s the same, though he has a bit more power in maintaining a more humanoid form, due to his heritage. Many demons have humanoid-esque “true” forms as well, likely because humans are a huge part of their lives. But there’s always variation, and different tastes.
So when Dipper feels more unhinged and less human? His form mirrors that. A surge of rage could trigger his eyes to revert, or the yearning for power (whether it be to destroy or protect) could trigger a full form shift. Feeling weak and helpless will often make him shift back to a childlike age.
He “ages” with some humans (usually Mizars), because he feels so deeply connected to them and the time they currently reside in. If they drift apart on good terms, he’ll slowly revert back to a young adult–considered the “prime” human years.
His gold blood? Most likely due to Bill Cipher. Dipper quickly becomes his own demon, but certain things from Bill’s design cling to him. (He tends to bleed red while feeling more human though!)

