uhh Hyo-un was a kid when he met taehyuk but Hyo-un is technically older than him. Hes just not physically developed nor mentally developed. Hes not human remember? His ability to grow and physically develop in his human body probably doesnt work the same as ours. Just assume hes a child around the age of 10, Taehyuk is probably around 21 so the difference is around 10+ years.
Spoiler
Yup he was brought to earth as a dragon or some supernatural being and copy the first thing that appear to him which was a human child
Technically hyo un could be any age hes not a human and wasnt born like normal but because the first human he saw was a child he picked that form
whats their age difference cus like holy shit