Drinking Age

Many people think if your old enough to fight you should be old enough to drink. Is this a weak analogy? I could see it being one because taking responsibility to protect your own country is very different from allowing an age group to do something that could unnecessarily harm them and especially harm others (i.e. car accidents). 

I would ask why. Why do you (does the person) think that if one is old enough to fight in a war, then one should be old enough to drink? Once we have a reason, we can evaluate the argument. Until then, we just have a claim.

Let's look at some possible answers:

1. Fighting in a war requires a certain level of responsibility, and that level is more than enough to drink alcohol.
2. Fighting in a war requires a certain level of maturity, and that level is more than enough to drink alcohol.
3. Fighting in a war requires a certain level of self-control, and that level is more than enough to drink alcohol.

The assumption here, of course, is that 18 year olds are responsible, mature, have enough self-control, etc. to fight in a war. Another possibility is that they are not these things, but we need people to fight, so we make do at the risk of harming those enlisted. But let's go with the former assumption. The level of responsibility, maturity, and self-control is adequate for X (fighting in a war), so does that make it adequate for Y (drinking)?

I see this as an argument where the situation matters. For example, in wartime where there is a dire need for soldiers, the level of required personal characteristics (responsibility, maturity, etc.) can be far lower than during times of general peace. If allowing 18-20-year-olds to drink has a massive downside and little upside for the community, the level of required personal characteristics could be significantly high. If one can show that the level of required personal characteristics for fighting is greater or the same as drinking, it would be a strong analogy.

You answered your own question.