The fact is the CPF Board (or the DPS carrier -- it's one of the two) sends out notices when members are first enrolled in the DPS. It's certainly possible Singpost couldn't deliver a letter, or the letter arrived but Mom or Dad dropped it, or whatever. No delivery system is 100% perfect....
....Which is precisely why the DPS is opt-out, not opt-in! The CPF Board cannot read minds! Hence, it has to make a decision, and clearly the correct decision is opt-out. Otherwise you'd have hundreds or even thousands of families with surviving dependents in more dire financial trouble because they aren't getting even modest life insurance payouts. Do we want more destitute children or some people who are paying modest premiums for coverage they don't need? Those are the only two choices here, and OBVIOUSLY fewer destitute children is the correct answer.
And there's nothing whatsoever hidden here, even if you miss the enrollment notice. When you log onto your CPF online account you can see premium deductions and you can also see your DPS enrollment status in the "My Messages" section. This information also appears in the CPF Board's yearly statements. And if/when you're making housing payments from your Ordinary Account you'll surely notice, right? The DPS premiums are drawn from the same source of funds (OA).