Liu Qianqian and Dawei Yihang are 7 years old.
What is the concept of 7 years old? It means that when Qianqian grows her first tooth and speaks her first complete sentence, Wei Yihang does not exist in any sense in this world; and when she has met the requirements for attending primary school for one year (6 years old +), he is still progressing hard on the road of evolution from a fertilized egg to an embryo.