What is the oldest thing? I'm well aware of the celestial bodies considered as the oldest visible in our galaxy.

My question might feel a bit odd: What is the oldest thing in the universe? I am puzzled with this question as the only answer that I have found by myself sounds weird.

As the notion of "old" only makes sense in an observable and ellapsing timeline, the most obvious answer to me would be "Time is the oldest thing". There is no such thing as a measurement of "old" until there is actually mass to allow time, so which predates which? I'm pretty sure the answer will tightly be linked to Planck's constant, but how?

What am I doing wrong? How could I find relief in an answer that wouldn't be just philosophical, but also logical? :)
Pretty much every hydrogen atom that's in a glass of water has a proton that dates from 1 / 1000000 seconds after the big bang.  That's older than the cosmic microwave background, which dates from almost 400,000 years later.