Hello and welcome!
As the other members mentioned, it is quite impossible to tell an age of a bird once it has it's adult plumage. Usually it is possible to make an estimation for very very young birds, or very very old ones, but even that's not a rule. Very smooth, non scaly/flaky legs could hint that it is a young bird, but not necessarily. This is applicable for all parrots.
For example, our budgie that was found over 7 years ago, was already an adult, but it is estimated that it was a young adult seeing her feet and beak was very smooth. She already had her adult plumage, but it is hard to say if she was maybe under a year, a year or maybe 3, or even more years. For sure she was somewhat young as she is still with us and still very active.
For GCCs they already get their adult plumage with the first moult which can be as early as 5-6 months. As younglings they might have more roundish eyes (baby eyes), and in other pyrrhura species which have prominent iris colouration (like the white eared or painted conures), the iris colour will start showing somewhere around age of 10months, even though they already have their plumage. With GCCs this is much harder as they have brown irises so it's not noticable.
Tl;dr - it is truly impossible to estimate the age of a bird.