The little old lady from pasadena
has a pretty little flowerbed of white gardenias

Jan and Dean get straight to the point. This old woman's "pretty little flowerbed" is her bathing suit area and the white gardenias are actually her white pubic hair

But parked in her rickety old garage,
There's a brand new shiny red super stock Dodge.

the rickety old garage is her pussy, and the brand new Dodge is a young, strong penis.

She drives real fast and she drives real hard,

she fucks like a beast, you wouldn't expect an old woman to be aggressive in bed but the little old lady from Pasadena is wild

She's the terror of Colorado Boulevard.
It's The Little Old Lady From Pasadena!
If you see her on the strip, don't try to choose her,
(Go Granny, go, Granny, go, Granny, go)

this is repeated throughout the song. Jan and Dean are encouraging her promiscuous behavior

You might have a go-er, but you'll never lose her;
(Go Granny, go, Granny, go, Granny, go)
She's gonna get a ticket now, sooner or later,
'Cause she can't keep her foot off the accelerator.

Jan and Dean are saying that she's gonna get in trouble for fucking teenage boys. She obviously can't help herself. lots of things could go wrong, she could get an STD, get arrested for statutory rape, get caught by her husband, get caught by their parents. Rumors fly, she could be ostracized from her church, her bridge group. she's playing with fire

And ev'rybody's sayin' that there's nobody meaner than
The Little Old Lady From Pasadena.
She drives real fast and she drives real hard,
She's the terror of Colorado Boulevard.
It's The Little Old Lady From Pasadena!
You'll see her all the time, just gettin' her kicks now,
(Go Granny, go, Granny, go, Granny, go)
With her four speed stick and a four - two - six now;
(Go Granny, go, Granny, go, Granny, go)
The guys come to race her from miles around,
But she'll give 'em a length, then she'll shut'em down.

she has a reputation

And ev'rybody's sayin' that there's nobody meaner than
The Little Old Lady From Pasadena.
She drives real fast and she drives real hard,
She's the terror of Colorado Boulevard.
It's The Little Old Lady From Pasadena!
The Little Old Lady From Pasadena,
The Little Old Lady From Pasadena

just driving home the point