I am playing on Normal lifespan with a dog in the household.
According to Carl's Guide, this dog's lifespan would be as follows:
Puppy Adult Elder
Normal 3 Days 24 Days Varies
In my Townie Dynasty, I track weeks/dates diligently because I schedule each household's free days around the new heir's birthdays.
This morning, I went back through my thread to note each time my dog received an Age-Down Treat.
Week 18/Thu (Day 131): Scamp the Akita puppy is adopted
Week 21/Thu (Day 152): Scamp ages up to elder (21 days from adoption to elder)
Week 22/Sun (Day 155): Scamp's 1st Age-Down Treat (3 days later)
Week 24/Thu (Day 173): Scamp's 2nd Age-Down Treat (18 days later)
Week 27/Wed (Day 193): Scamp's 3rd Age-Down Treat (20 days later)
Week 30/Mon (Day 212): Scamp's 4th Age-Down Treat (19 days later)
Week 32/Sat (Day 231): Scamp's 5th Age-Down Treat (19 days later)
Week 35/Thu (Day 250): Scamp's 6th Age-Down Treat (19 days later)
My doggo has received an Age-Down Treat as soon as he's seen ageing up.
In my case, the interval between Age-Down Treats has stayed consistent at 19 days average, with just one day more or less.
Anyway, I went and calculated this to satisfy my own curiosity but so I thought I'd share.
(And yeah, I'm a nerd, but you knew that already. lol)