How are people getting from "pictures on facebook indicate teens were probably drinking" to "drinking and driving"?

I was just on another thread a few days ago about a 19 year old who was thought to be drinking and everyone was talking about drinking and driving.
Why is everyone assuming that drinking under age 21 = driving while drunk? And why doesn't anyone seem to put together drinking while over 21 with driving while drunk?
I just found this information on MADD's website:
In 2002, 2.3% of Americans 18 and older surveyed reported alcohol-impaired driving, including 3% of 18-20 year olds and 4.1% of 21-34 year olds
That would seem to indicate that those of legal drinking age up into the mid 30s are
more likely than older teens to drink and drive. So why is it that when a 30 year old DISers talk about going out for drinks other DISers simply post the drinking smily, but when an 18 year old is said to be drinking right away drunk driving is brought up?
In my experience, the people I see taking risks when it comes to drinking and driving tend to be older adults. I had friends who drank in high school, but none of them (as far as I know) ever drove afterwards; they specifically arranged to be drinking at the home where they would be spending the night. I didn't drink in high school, but did drink long before I was 21 as did almost everyone I knew in college. Since it was a residential college there was really no need to drive anywhere. "Home" was never more than a 10 minute walk away. I suspect per capita drunk driving rates for residential college students (while they are at school at least) are much lower than they are for the general population just as I suspect there are lower per capita rates in cities with fantastic public transportation.
My 55 year old father will go to a wedding and down five or six drinks in the first hour and a half and then leave after three hours. Is he within the legal limit for driving? I don't know and neither does he. A similarly aged friend of his was recently arrested for a DUI. The friend came to visit my dad and told him the story about getting arrested well downing numerous glasses of wine. Then, after clearing having had enough to drink to be over the legal limit he got in his car and drove home! (My mother did yell at my father for letting him go and my father's excuse was "Well what was I supposed to do? It's not like I could drive him since I was drinking too"

)
Another distant relative is in his 80s and has just in the past few months been involved in 3 traffic accidents after he'd been drinking. The first two accidents didn't involve any other people, but the third one involved him going over the median and hitting another car. He was not hurt and luckily the other driver was sent to the ER but checked out fine. Amazingly, he was not charged with a DUI or even asked by the police if he had been drinking.
Of course this is anecdotal, but I think that these cases are a result of the tendency for people to associate young people with drunk driving; the corollary to that association is that older people are thought NOT to be associated with drunk driving. Apparently the police found it inconceivable that a man in his 80s could be driving drunk and so my relative retained his license after the accident. My father insists his method of essentially binge drinking for an hour and a half and then drinking nothing for the next two hours leaves him perfectly fine to drive; after all, he'll say, I've been drinking for almost 40 years, I know my limits by now. Of course, he doesn't mean by that he knows how much alcohol he can have before he reaches the
legal limit, but rather that he "knows" how much he can have before he reaches some limit he has determined for himself which is surely above the legal limit. But, you know, he's 55 and it's only teenagers and college students who drive drunk, so there's no need to worry about him.
Drinking and drinking and driving are two very different things. (Though, even if it could be shown without a doubt that a student was drinking and driving, I still don't think that should have any bearing on what goes on in school/school activities.) We all recognize that when it comes to people over age 21, so why when it comes to those under 21 is it assumed that they are one in the same?