How strong is the kingpin on log trailers and does age of log trailer weaken the kingpin?

No one will give the strength of a kingpin, but assume it to be at least as strong as a Grade 8 bolt, whose minimum yield strength is 130,000psi and whose minimum tensile strength is 150,000psi.

The age of a trailer doesn't weaken the kingpin.

However, most log trailers connect by pintle hitch rather than kingpin.