Keep in mind however, that many young unbeliever “come home” later in life, especially after they begin to raise families. Also in the past many actual non-believers probably claimed nominal Christianity (but lacked real faith) for social reasons that are less prevalent now. The reality may not be nearly as dire as statistics make it seem.
Well, as to your first point, when they surveyed older age groups in later years they found the number of non-believers remained remarkably static. I’m sure some became believers, but either an equal number gave up their faith or the numbers are not that significant overall.
Perhaps that may be different with the next generation, but there is little sign that it is likely to happen.
As for your second point, the professional pollsters do their best to get accurate data, but it’s true there is no guarantee they will get honest answers. But I suspect that the trends they have found are real enough.