This Pearls Before Swine strip ran May 15, 2016. Since the humor in this sequence depends on the element of surprise,
I reserved my comments until after the reveal because that's where the payoff is. All the information we get beforehand - cruel treatment, depression, lost love -- pop into place once we see that it's Charlie Brown.
The last panel has all the comedic gold.
"Good grief, Charlie Brown" in the Peanuts strip is an expression of exasparation (as in, "Good grief, Charlie Brown, you're such a blockhead!") Goat's delivery would have been different, as he was stating a discovery. "Good grief, [it's] Charlie Brown."
Charlie Brown's comment about middle age is an understatement. Peanuts started in 1950, with the apparent age of the characters being 6. So in 2016, Charlie Brown would either be 70 (his presumed real age), or 64 (from the start of the strip). "Middle age," indeed!
As a coda, Rat brings in an old Peanuts trope in keeping with his cruel nature (and perhaps in the process pointing out the inherent cruelty in the original running gag). Another masterwork from Stephan Pastis.