cancelled. It happens.
I actually had my 13 year old act as the chaperone for her 7 and 10 year old siblings this summer and it cost us nothing.
haven't been there for years. Is it reasonable for a kid to make a connection on Southwest?
On my prior note, I trust my son to make connections but I would avoid certain airports, namely O'hare and Atlanta.
There are basically two concourses (plus a tiny Concourse C), each of which is just a long line of gates. The only moderately difficult part of making a connection would be if his arriving flight was in Concourse A and his departing flight was in Concourse B (or vice-versa), in which case he would have to walk towards security to the food court and then walk over to the other clearly marked concourse.
I don't know any 13-year-olds so it is hard for me to gauge what they are capable of, but a connection in Midway is much easier than in O'Hare or Atlanta (but harder than in a small regional airport, of course). The link has a map of Midway gates.
(edit for the SWA update...didn't realize they allow unaccompanied minors for free over Age 12)
While your son may be entirely able to navigate an airport on his own, the airlines choose to err on the side of caution and ensure all minors are accompanied end-end.
It helps minimize the inevitable outcry that can break out on social media and only amplified when children are involved. The airline has a direct interest in ensuring your son safely makes it to his final destination.
The UAM fees also ensure your son is properly supported if his flight is delayed or cancelled. Think of how difficult it is to rebook a delayed/cancelled flight at the airport as an adult. This gives your son some much-needed support if that happens.
Generally, I've seen the cutoff being 15 (SWA being an exception), which also aligns with airlines allowing minors into Exit Rows.