Classic symptom of Marek's Disease (first one leg is useless--no pain) Then, usually wing on same side droops, then other leg goes and bird can only shuffle around--will eat and drink, if if can readh food and water--again, no pain. Death most often occurs, sudden death or after a week or so unable to walk. HOWEVER, this being said, hope it is NOT Marek's! Cockerels most often show first symptoms at about 2 months, pullets, usually, at about 4-5 months or when just ready to lay or after first egg or 2.. However, it can occur at any age (but not as baby chicks or not under 2-3 months).
Few recover, and recovered pullets may lay normallly--it is a long lived virus but is not carried in the eggs----but if they do recover, they are carriers of Marek's for life, and all your birds are or will be exposed. Not all display symptoms, and some may be immune. It is worldwide--dreadful for loss of birds--and I hope you do not have a bird with Marek's. I did not know what it was when my first bird died of it (sent to me as an "immune" hen and went through the symptoms before dying and fortunately not a virulent case, as I had no more birds come down with it for another year)--but then lost about 6 young birds. Now, I vaccinate every chick at hatch--and for 14 years, have had no more Marek's. I NEVER purchase birds from individuals OR hatcheries that are not vaccinated at hatch! Yet, happily, some poultry people never see it at all! And that is GREAT management of poultry!!
Good luck