Lichen planus is rare in children and is usually seen in adults aged 40-60 years old. The lesions are usually highly pruritic and have a slightly scaly surface. They are often shiny flat-topped papules. After the papules flatten (can take a few months) the patient is often left with hyperpigmentation.
It can affect many areas of the body but commonly the arms, legs, trunk, mouth, groin, or scalp.
It is not infectious.
Lichen planus normally self-resolves after 12 months. Topical steroids can help relieve the itch.
References
https://www.nhsinform.scot/illnesses-and-conditions/skin-hair-and-nails/lichen-planus
https://www.bad.org.uk/shared/get-file.ashx?id=168&itemtype=document