This summer camp is suitable for 8 to 10-year-olds.

year-olds 是由名词 year（年、岁）和形容词 old（老的、...岁的）组成的复合名词，加上连字符和复数后缀 -s 构成，表示"...岁的人"。8 to 10-year-olds 指"8到10岁年龄段的孩子"。