At what point...

in life do people suddenly decide that a work shirt, chino shorts and suede loafers with no socks is a cool summer look?

maybe when they are in a conflict between warm weather and safe for work clothing? half "I don't care" and half "I should care a little bit"

No these are people wandering around at the weekend with their kids.

It seems to happen after the second kid arrives (or as soon as you turn 21 if you went to a minor public school).

I’m going the other way. I bought a baseball cap the other week. The first such since I was a teenager. I also own an identical pair of Nike Air Max to those I had aged 14. 

maybe people don't think "it's a cool summer look" but rather "I can't be arsed but don't want to stand out negatively"

i.e. it's not thought to be a "cool" summer look, just "a possible summer look"

As a German I half expect you to wear birkenstocks with socks during the summer.

I've recently taken to this summer look: straw hat, linen shirt, pink shorts, loafers

Pink shorts should be classified as some sort of crime against humanity.

My bro has a pair of pink shorts with little t-rexs all over them in the same manner you sometimes see shorts with an anchor or fish pattern.

To the OP:  at about the age of 45, I think.

 

3 years to go...

Fat Face is the way to go if you want casual cool.