https://www.nhs.uk/conditions/klinefelters-syndrome/
'Symptoms of Klinefelter syndrome
Klinefelter syndrome does not usually cause any obvious symptoms early in childhood, and even the later symptoms may be difficult to spot.
Many boys and men do not realise they have it.
Possible features, which are not always present, may include:
in babies and toddlers – weak muscles and very flexible (hypermobile) joints, learning to sit up, crawl, walk and talk later than usual, being quieter and more passive than usual, having undescended testicles, only one testicle, or a smaller penis
in childhood – shyness and low self-confidence, problems with reading, writing, spelling and paying attention, mild dyslexia or dyspraxia, low energy levels, and difficulty socialising or expressing feelings
in teenagers – growing taller than expected for the family (with long arms and legs), broad hips, poor muscle tone and slower than usual muscle growth, reduced facial and body hair that starts growing later than usual, small, firm testes, and enlarged breasts (gynaecomastia)
in adulthood – inability to have children naturally (infertility), low sex drive, small, firm testes, and erection problems.'