Puppy with older dog

I have a 15 year old Aussie who is quite calm in her older years and may be around a few more years if we are lucky.

I have been considering getting a 4 month old puppy to add to our family, but I have never raised a puppy, as all my previous dogs were adopted as young adults (age 2 ).

Can anyone give me an idea of what’s it’s like raising an Aussie from 4 months and what I might except between my 15 year old dog and a 4 month old?

Thanks.