Hi Julie
I did the previous version of this trip (with two days on the Rocky Mountaineer) a couple of years ago in the first two weeks of September. I've done three other JY trips since but Canada remains my all time favourite - it is breath taking and you will have an amazing time.
In terms of the weather and clothes to take - layers are the key. When you are in the Rockies themselves - especially on the journey through the Icefield parkway - it will be pretty cold, especially in the mornings (we had snow on 3rd September). I definitely recommend taking a hat and gloves, thin thermals, fleeces, a waterproof jacket and sturdy walking shoes/boots. However, by the time you reach the coast, it is likely to be much warmer, but possibly a little bit more rainy. My group were in t-shirts for most of that part of the trip.
As for ages, and as with every JY trip, you will have a wide range and 60 is definitely in the middle. On my recent trip to China we ranged from 40-70 but for Canada I think we were 30-85 - and the 85 year old came enthusiastically on the whale watching excursion in Vancouver with us!
Happy to help if you have any other questions. I would certainly consider returning to Canada with JY in future but next year, I've got my eye on Nepal!
All the best
Mandy