What if dual nationality is acquired before the age of 20?
Not picking on you, since you probably already know, but dual nationality by birth (acquisition of that dual status without volitional/conscious intent) is allowed to continue indefinitely.
Yes, even for this smallish group there is worry or concern, or the suggestion or claim that "You have to choose by 20". (or is it 22?) But our kids are in their 30s now, and they still have--and have renewed--their j-passports and another, maybe even two j-passport renewals after turning 20. One of ours had a high school teacher that irked us a little by bringing up to this kid the 'necessity' of choosing more than it was their business to be doing so. (Not their business
at all, really.)
One very strong suggestion: Don't lie about having a second citizenship/passport on the Japanese passport application or renewal form (a very official document, like a tax return or MyNa card application). There is a check box on the form (this info is dated, so it could have changed) that asks if you have a second citizenship. Lying could be grounds for revoking one's j-citizenship (or some other penalty). Be honest, and if they ask, just say you haven't decided yet. For folks like this the law is simply suggestive and fuzzy, something to the effect that you will "try" or endeavor" to relinquish one citizenship or the other, and not that you actually have to.
Separately, there are a few countries that do not allow a person to relinquish/drop citizenship. From past reading, Japan does accommodate these folks--allowing them to become Japanese, while making an allowance for the other country's citizenship laws.