All students are funded to 19 anyway for a-levels etc, so if a student follows a normal program of study, e.g. two year A-levels, they will be fine at post-16.
My understanding is that the issue may come if they e.g. want to repeat a year of sixth form. There's a small minority of students every year who either restart Y12 with different subjects, or repeat Y13 to try and improve their grades/gain passes. If a student were already 19 I don't think there is funding for this- that would be where they might lose out. They'd have completed the program of study, so the above statement doesn't apply.
It's also worth bearing in mind FE colleges aren't bound by the admissions code in the same way schools are, so could, in theory, reject an 18yo student wanting to restart courses due to lack of funding.
Say, for example, they do a year of A-levels at secondary school, decide it's not for them, and want to do something vocational e.g. a T-level at college. If they're already 18 at that point, without an ECHP, the college could choose to reject them.
It's not something I would base an overall decision to defer on, though, because it doesn't apply to most students, and there are other routes that would be available.