You mean the Mustangistas⁽¹⁾ have a random perception of calendars? What are the "official" dates, for them?

That said, it's Chevrolet that is talking of a 50th birthday, here, not Ford
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(1) I assume this stands for "Mustang enthusiasts"

I have no problem with calendar years, but Mustangistas do. Except when they want to have a party.

Wikipedia:
"The Ford Mustang was brought out five months before the normal start of the 1965 production year. The earliest versions are often referred to as 1964½ models, but VIN coded by Ford and titled as 1965 models with production beginning in Dearborn, Michigan on March 9, 1964 and the new car was introduced to the public on April 17, 1964 at the New York World's Fair."

Doesn't that means 50 years + 4 days, today?

(Source, for the "1964½ model" name: Tom Corcoran's "Mustang 1964 1/2-1968" book)

Anybody else confused how they can officially celebrate the Mustang's 50th birthday in 2014??

"Du plaisir à conduire" ?
Comme tu le dis, tout le jeu de mot est sur le "bringen", très bien rendu dans la version anglaise (trop facile, les anglo-saxons ont la même origine linguistique ).
Traduttore, traditore, comme d'hab

par contre leur traduction est toute foireuse.
Le plus proche serait "Même une Mercedes peut vous apporter le plaisir de conduire" (pour tenter de garder le jeu de mot).

Must be a real picture. Or the Munich BMW Museum(fr) has a fake one too

One of my favourite "rivalry ad" is this one:



(not sure it is a true one, though)

Camaro wishes Mustang "Happy Birthday!"



My favorite picture of both cars (click...):