Although Zelda's age and physical appearance changes with each game that is released there are certain elements about her overall design that remain the same, making her easily distinguishable as the beloved princess Zelda of Hyrule and iconic parts of her character. I will go on to explain what those are.
First up is her hair and eye color. Her eye color has always been some shade of blue and her hair color has been most commonly blonde––with the exception of a couple of the earlier games, and Twilight Princess where she was known to be brunette instead. He hair is always long, usually extending to just above her butt, and is usually down, but sometimes has been seen partially tied up or braided. She always has two parts of hair that extend down by her face, and sometimes has bangs. Her ears are long, likely due to being Hylian, and she is always seen wearing earrings––most commonly having the Triforce shape on them––and wears a tiara, oftentimes with a red gem in the center of it (though there are a couple games where it has been blue instead) which spans across her forehead.
The clothing she wears are known to change with each game as well, but various things about them appear to remain fairly consistent. For one, Zelda is always seen wearing a dress, of course I think that's a given for a princess, but yeah. The colors often seen on it are white and pink, and sometimes even a light blue color is added, mostly for extra decoration on the dress or sleeves if the dress has them. She wears gold-colored shoulder pads with a decorative ornament hanging across her chest, and she also wears a gold belt––sometimes with other gold ornaments hanging from it––always with an blue apron hanging from it with the crest of the Hyrule Royal family embroidered into it. She'll often be wearing light-pink or white gloves which go up a little over halfway her upper arm. Other times she'll be wearing gold cuffs on her wrists instead. Though it is usually hard to see, as it is not normally visible, she can also be wearing either high heels or boots, both of which complete the look for Zelda.