1.Wow! Did you see that teenager run that stop sign? Teenage drivers are really pathetic.

what type of fallacy is this?

hasty generalization

Ad Hominem (Guilt by Association)
argumentum ad hominem

(also known as: association fallacy, bad company fallacy, company that you keep fallacy, they’re not like us fallacy, transfer fallacy)

Description: When the source is viewed negatively because of its association with another person or group who is already viewed negatively.

Logical Form:

Person 1 states that Y is true.
Person 2 also states that Y is true, and person 2 is a moron.
Therefore, person 1 must be a moron too.

It may not be a fallacy at all, if (left unsaid by the 'accuser', but factored into his statement) awareness of documented statistical evidence exists sufficient to make this point factually, e.g., that teenage drivers have a substantially higher accident rate than other demographic/age groups.
In this event (& I have no idea whether this is the case) the presumed fallacy fails, in light of additional evidence known to the accuser.

I think the broader point is that at best, this a WEAK fallacy, either ad hominem or hasty generalization. Let's take the counter argument i.e., that the
accuser had no such statistical knowledge, implicating teenage drivers. He may simply be speaking from his EXPERIENCE as a driver, and may have had
a series of bad encounters with teenage drivers. In this case he is voicing his EXPERIENCE-BASED OPINION. Yes, you can argue presence of a fallacy, but the strength of any fallacy is directly proportional to the degree to which LOGIC becomes absent or violated. The mistake I've seen made most frequently on this Forum is to confuse an OPINION, even a demonstrably factually incorrect opinion, with the necessary presence of a Logical Fallacy. Not the case!

Over-generalization. Many teenage drivers are indeed inexperienced and aggressive, but it is unreasonable to draw the conclusion that this applies to all teenage drivers.