Is it acceptable to wear shorts (onsen)?

I would like to take my parents to an onsen when I go to Japan but she wants to know if it is acceptable to wear shirt/shorts. If not, what is the appropriate attire for men and women and does it also apply to teens?

Thank you.

If you mean wearing shirt/shorts around town then that should be fine, weather permitting of course. If you are staying at a high level ryokan you may want to dress more like if you were staying at a top level hotel, but that would generally be a personal choice. The baths on the other hand are to be enjoyed naked except for the rare exceptions where bathing suits/towels are required. These rules apply to both teens and adults.

Onsen is basically gender separated & to bathe naked. Nobody minds.
Some mixed gender onsen may offer(sell) special bathing attire that looks like a oversized moo moo.
If too shy, find in-room onsen(expensive) or time reserved kashikiri onsen so they can bathe together in privacy. They are becoming more popular and many are free to reserve about 50min.

If you are talking about what to wear when you wash and actually get into the water, the answer is as above, nothing. There are no exceptions to this- people of all ages take hot springs naked, and wearing anything into the onsen is strictly forbidden.

If you or your parents are not comfortable with this, you will have to find one of the few places where swim suits must be worn, for example Yunessun in Hakone.

Anyway, Do not put the towel into the bathtub, especially public bath.

JiHwan,

There are many people who are not accustomed to being naked in front of others. However, being naked in an onsen is very much the norm and since most onsens separate the sexes it is less of a problem for the shy.

Keep in mind that when you are physically not in the onsen, it is proper to cover your pubics in public so the towel provided serves that purpose.

I say that you keep the old saying in mind...When in Rome do as the Romans do

Or when in Japan...

''There are no exceptions to this- people of all ages take hot springs naked, and wearing anything into the onsen is strictly forbidden.''

Is this really true or there are some places you can wear swimwear?

Jeron,

Is this really true or there are some places you can wear swimwear?

Yes, it is really true.

The only places where swimsuits are allowed are family spa resorts like Yunessun in Hakone (already mentioned above) and Hawaiians in Fukushima.

be aware yunessun is nausea enducing and not somewhere you want to be unless you have kids.

I went as someone gave me a couple of free tickets they couldn't use. It was disgusting people bathing in curry flavoured bath water that stinks and wine flavoured baths and ramen flavoured baths.
There were so many people in the water at once you cant even move it looks like a scene titanic or something with so many people in the water.
All the chemicals in the novelty flavoured waters made me itchy so I couldn't enjoy it even the more peaceful "tradional style" areas (where I think it was no bathing costumes)
but even that was a bit fake and tacky. I suggest something a little more authentic if you plan on enjoying the onsen experience. Cant you just cover your bits with a little towel or two?