Previous chapter implied he was a generation above (calling her niece). So 15-30 years. Standard Chinese logic of cultivation means age gap becomes meaningless. Though personally I feel that only applies when there is actually age extension (which douluo doesn't have) and both parties are well into adulthood. which Qian Renxue is not, even with bs cultivation early maturity.
Hejoro
ch 0 9 Chapter 9 She is indeed the daughter of a strong man who escapes the world! Abnormal talent!
Douluo: My daughter went down the mountain and exposed Me to The World
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