Polymer clays or plasticine clays are made of synthetic plastics and require âcuringâ or low heat to harden. They stay moldable for a very long time if stored properly in a plastic or metal box (no paper or fabric that would absorb the moisture). Polymer clays are usually very smooth and have a silky feeling.
On average, 4th graders are 9-10 years old because that is the normal age for the grade. But kids could potentially be anywhere from about 8 to 11 years old, since some kids hâ¦ave to repeat a year in school and get behind, and then some kids are a year ahead or have late birthdays before the cut-off date.
What age do kids start fifth grade? Since kids generally start kindergarten at age 5, then they  would be 10 when they start 5th grade.     But, this can vary, depending on their birthday, the cutoff daâ¦te  for starting school in their state, and whether or not they ever  got held back.