uncle wang is only 35 years old at that time哪里有错？

at that time用过去时

is改为 was

at that time是在那时的意思,所以用过去式

uncle wang was only 35 years old at that time

is 改成 was，从at that time 可以看出时间是过去的。

is 改成 was uncle的U应该大写
uncle wang is in his thirty-five.

is 改成 was因为at that time 是过去时态
uncle的U应该大写