Choice (A) does not weaken the OA. If choice (A) were NOT true (i.e. "Drivers 65 and older DO, on average, drive very many fewer miles per year than drivers 24 and younger."), then that would indeed weaken the argument. Thus, we need to assume that choice (A) is true in order for the argument to hold.
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I might not be an expert, but will try to illustrate how i interpreted the question.

< 21 - 16%
21 - 24 - 11%
.
.
>65 - 3 %

Conclusion: >65 year olds are better in driving than younger people (all <65 are younger people i.e. 97% accidents, so even if few age groups have less than 3% we need to check on an average, the trap is that younger means not the specific people but on a whole average) because of experience.

A says that : These guys DO NOT drive FAR FEW miles than younger people. (if they drive far few miles then this conclusion crumbles saying you can't take an accident rate and judge them, saying that they are making LESS accidents because of experience. If they drive far few miles, It makes them(>65 year old) less prone to accidents.)

E : though at start i opted this option too. what if there exists an age group 35-40 that has less than 3% accidents. So what that 97 % is averaged to 20-65 year old, who have 97% accidents, on average do more accidents right!.

So A is best over E.

Answer should be A.

No wonder this is OG question. OG Never forgets to entice us with each and every question.

Note: Sorry, I just woke up so please be lenient on my English on this post. I tried to correct some grammar errors but am still sleepy.

Conclusion: Older drivers are safer than younger drivers.
Premise: A smaller percentage of older drivers are involved in serious accidents.

Because this is an assumption question, we know that the correct answer, when negated, will undermine the conclusion.

(A)- Negated: Drivers 65 and older DO, on average, drive very many fewer miles per year than drivers 24 and younger.

Well, if the older drivers are driving many fewer miles than the younger drivers are covering, then it's not necessarily the case that older drivers are avoiding accidents because they're driving more safely. Rather, they're avoiding accidents because they're just driving less. Because A, when negated, undermines the conclusion, we know we're looking at the correct answer.

(B)- Negated - Drivers 65 and older constitute a significantly larger percentage of licensed drivers in Wareland than drivers ages 18-24 do. ---- First of how much significant is really going to matter, moreover what if 3% of that exceeds the number of young drivers who met serious accidents

Between A and B, A seems a better choice. Rest choices can be eliminated very easily.
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Simple strategy:
“Once you’ve eliminated the impossible, whatever remains, however improbable, must be the truth.”

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My Notes:
Reading comprehension | Critical Reasoning