Have you considered having slightly different dresses for the two older, and 3 younger bridesmaids? My uncle/auntie didi this, as we were all aged 11-14 except one who was approx 5-so she had a younger dress. infact she had a shop brought ivory dress to colour match the brides, and we all had deep reds to match the wedding colour theme. You could also see if anyone could make the dresses. My mum actually made all the bridesmaids dresses except the 5 yr olds. She brought a simple pattern (trendy with empire line to flatter us all!) then adapted to fit each one of us perfectly. Good luck