Many of the congenital myopathies can present with similar clinical features.  Several vary in severity depending on age of onset:

Infantile Forms

May present as "floppy baby syndrome."  Severe weakness.  Decreased spontaneous movement, followed by delay in meeting motor milestones.  May have rapidly progressive weakness.  High risk of death due to respiratory failure within the first year of life.

Childhood Forms

Less severe.   Nonprogressive course.  Proximal weakness, hypotonia, delayed motor milestones.  May have myopathic narrow facies, high arched palate, kyphoscoliosis, pes cavus deformities.

Adult Forms

Often mild, nonprogressive weakness. 

References:

Katirji, B., Kaminsky, H.J., Preston, D.C., Ruff, R.L., and Shapiro, B.E. (Eds.) (2002). Neuromuscular Disorders in Clinical Practice. Butterworth Heinemann, Boston. (ISBN:0750671696)  Pp. 1114-1127.