To take a job working with year 9s

Unequivocal no. Also, your assumption is incorrect in my experience - the behaviour of the girls is no better, they're still as disruptive as the boys.

Having worked in a senior school as an LSA across years 7 to 11 I would say there will be, or there were when I was working, disruptive, noisy, mouthy, kids in every year, boys and girls. Plus as a support assistant not only was I put with the struggling children who were supposed to be my focus, but also those who had no intention of doing any meaningful work if they could help it. Social Media wasn't as intrusive when I was there and that won't help.
If you are only being employed for year 9s there will be no break from them either. Having said all that I loved my job and my memories are mainly good.

If they’re settled, grown up and mature, why would there be a need for the job role you’re considering?!

Why do you think all.of this should have stopped by year 9. Chatting in class, back talk, silliness can continue up through 6th form. Some kids might settle dpwn but those whp are prone to theses behaviours are unlikely to ever change

No

year 9 is generally the hardest year.

year 7 take a term or so to settle and then are ok.
year 11 are super stressed about exams so lots of exam related issues and anxiety.
year 8 begin to be difficult as they feel like they know the school and begin to get too big for their boots.
year 10 have just started GCSEs and are constantly told how important they are which keeps a bit of a lid on behaviour for most, plus they mostly chose their GCSEs so are out of the lessons they hate,

year 9 is peak obnoxiousness and they are not yet getting the “work hard GCSEs are inportant” message plus they have to be in subjects they hate.

Christ y9 the worst year IMO im ex secondary school teacher

No experience in teaching personally, but I remember being in year 9 (more years ago than I'd like to admit), and it was the worst year of being in school. I remember the teachers would say that even the nicest year groups seemed to flick a switch when they started year 9.

Years 10 & 11 were when the maturity and being kinder started to kick in when I was at school - a different uniform to the rest of the school, we were doing our options and felt like we had a little more control and we all wanted to be an adult

Will they be a calm, well developed age group to work with?
Short answer, No.

Sorry to be the one to say it, but from experience, y9’s are the worst!