British citizenship for children under 18

I am asking on behalf of a friend and I appreciate all the information I can get!
A woman, whose husband died in her country of origin was an asylum seeker here in the UK and has brought her two children to England about a year and 6 months ago. Just a year before they came, she became a British citizen by naturalisation. The children are now 15 and 17. She would like to apply for citizenship for them but:
1 -> is there a requirement to how long they will have to live in the UK before they can apply under form MN1(Application to register child under 18 as British citizen)?

2 -> Are both the 16 and 18 year old filling a separate application MN1 form, and is the mother able to fill on behalf of the 17 years old as well?

Many thanks!

Maybe this can help:
http://www.immigrationboards.com/britis ... ml#p592355