To be fair, a lot of old PG movies were only PG because PG-13 didn't exist.
There you go.
And too, if you ask me, ratings seemed a little clumsier back in those days too... I've seen some older R-rated movies (like some of those really old Charles Bronsan movies) that probably would've only gotten PG-13 today... I heartell back in those days, half the time movies were given R ratings was simply because they were longer than movies typically are (hour and a half to two hours).
But then again, even more recently, some ratings are still clumsy... again, look at KANGAROO JACK, that only got a PG rating and sold as a "kids' movie" simply because they added more scenes with the kangaroo because test audiences were apparently only interested in the kangaroo: it otherwise has language, violence, sexual content, and adult humor... it should at least be PG-13.
But as far as Mel Brooks goes, let's face it, his comedy has always been more on the adult side, though his were usually done in somewhat good taste, if only half the time (BLAZING SADDLES had some arguably offensive humor), but the thing about Brooks is his were genuinely funny... most similarly adult or raunchy comedy today is simply lowest common denominator humor, writers just writing what pops into their mind, going for the easiest and cheapest laughs they can get.