system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why causes particles to decay? And why does it take so long for some?

Some elements for example can take millions of years to decay. Also, how can we tell their age?
gpt
The stability of an atom is determined by the interaction of several different systems

1. The Strong Nuclear Force. 

This holds the nucleus of an atom together by binding the constituent parts in ways that are incredibly complex, but it's very strong at super small scales.

2. The Coulomb Force.

Thus is the positive-positive repulsion that is felt by the protons in an atom. This is the force that dominates when you go a little bit further away than where the Strong Force applies.

3. The Weak Nuclear Force.

This is the cause of the actual process of decay. This force facilitates some particles changing into others. Sometimes, those particles will make the new atom unstable, and it decays immediately.

So the overall effect is that large atoms begin to exit the size for the Strong Force to apply, and some slight change due to the Weak Force causes it to go unstable, at which point, the Coulomb Force shoots out something at great speeds.

We can tell object ages with decay because while nuclear decay is a random process, it follows a strict formula that half of whatever sample you have will decay in the same period of time. This is true of any size sample. This time for half of the sample to decay is called the "half life", so if you know the sample size now, you know how much there should have been, and you know the half life, you can calculate the time to get back to the original sample.