Regarding Passport validity for British Dependent Visa

Hello,

I'm planning to apply Dependant visa (Dependant of British Citizen) for my wife and son
I was on Tier 2 visa earlier and my wife and son were in the UK as my dependants. My wife got sick and they went to India in Feb 2019. By the time she felt ok, their visa got expired. Then the pandemic started and I cudnt do much.

Now, my son is 17 years and 4 months old. His passport is valid for another 8 months (expires a day before his next birthday)

Questions:
1) Is it ok to apply for the visa now given his passport expires in 8 months time
2) Is there any rule that passport must be valid for x months before applying for visa
3) The guidance says it will take 12 weeks for processing the applications but I read it takes more time for a few people. Give this scenario, what if I apply now and my son turns  18 during the visa processing.

Appreciate your time and help.

Thanks

Can anyone / moderators advise me on this matter?

Appreciate your time and help

Thanks