Youth sizes in clothing are based on age.
However a child's or youths individual growth and build can make it necessary to go up a size or down a size in clothing.
Youth clothing sizes often go by the child's age and corresponds to a child's age.
For example sizes such as 2T are for toddlers and 4Y and 6Y are youth sizes.
For babies the sizes of clothing go in months such as 10 to 24 months and after 24 months it transitions to the toddler sized clothing.
Children grow at different rates though so you have to keep that in mind when buying clothes for kids as the kids actual size and build may not always perfectly fit within their age.
When kids clothes are too small for them you will know by the fit and comfort.
When kids clothes are too small they can cause discomfort, lead marks on the skin, leave indentions on the skin or even limit movement.
Or when clothes roll up or sleeves of shirts are too tight and cut off circulation.
If the kids pants are too small they will fit too tight around the waist and may not close around the waist or may leave red marks around the waist.
Any red lines or irritation where buttons are snaps are located are signs of clothes being too small.
For babies if the baby seems uncomfortable or is squirming a lot it can mean that the clothes are too tight.
Proper fitting clothes that are not too tight should be easy for the child to get on and off and they should also have full range of motion and should not be restricted in movement.
If the clothes restrict the child's or kids movement then they are too small and you should size up on the clothes.
It's always best to get a size a bit larger than your kid normally wears as they will grow into it and it make the child more comfortable.
Also different brands of clothing just like different brands of diapers can have different sizing and fits so you should check the size chart of clothes for each brand of clothing.
A shirt that is too small for a kid will be hard to put on or take off and can lead red marks around the neck or the head won't fit through the neck hole easily.
If you can't button up pants or button up shirts then they are too small.