What is the right 'age slab' for a 1grade child for CBSC? Which is the toughtest board, CBSC or ICSC?

My daughter is 6.3 yrs old (6years & 3 months). She is appearing for her 1grade final exams in CBSC school. What i have noticed is her clmates & friends who are in Gr 1 are already 7 or 7+ yrs old. Also Looking at her portion, i sincerely feel that it is too much for a 6 yr old. Please help me with your kind advise whether should i take a decesion of repeating her in Gr 1 next yr so that her age will be appropriate to take that level of studies or let her continue? Just for additional info, she is good in her studies right now. She hs got Silver card in 2nd sem. I am only worried that further grades should not be hard on her.....just a motherly concern. Please helps me friends!