Thinking of Emigrating but it could be complicated

I am married and I have 3 children from my previous marriage. 

1 son aged 19 and 2 daughters aged 17 and 12 who all live with us in England.

My youngest daughter has Downs Syndrome but is very healthy and thriving. 

My Husband has two brothers who live and work in Australia even though they were born in England, and have done for over 20 years.  His mum also lives there, she is on a bridging retirement visa.

Just trying to find out if we would be eligible to emigrate as a family.  My son is intending to go to Australia in Nov 2018 to travel and work for a year.  He is also going to enquire about a skilled visa as he has qualifications and experience as a Lawncare Technician here in England.  My 17yr old daughter is over halfway through a 2 yr “A” level Vocational Course.  She would like to join the Military or RAF (Military Police).

Various issues potentially;

Our age (I am 51, my Husband 48)
The fact our daughter has Downs Syndrome
Our eldest two children will be adults by the time we apply.  Do they have to apply in their own right?

So many questions......