Joseph,
Right this minute, I'm presuming YOU are regenerate. Am I wrong to do so?
The term "presumptive regeneration" is today simply inseparable from a particular Kuyperian-Dutch tradition. That particular strain seems to have led to a moribund membership of frankly unregenerate people. In other words,, it was so presumed that it led to presuming that conversion just happens, and means are immaterial. It led to an atitude of "guaranteed salvation" as Richard Z put it.
There is a sense in which "useful" terminology has been hijacked. There is a "simple" presumption, and there is "high-handed" presumption. To presume something in the simple sense is simply to believe a proposition based on limited or insufficient evidence. Was it improper for Stanley to say: "Dr. Livingstone, I presume"? I doubt it. There might be warrant to presume something even when evidence for it is lacking--for example if there is no counter-evidence, where it would be expected in case the opposite of the supposition was true.
On the other hand, "high-handed" presumption (my own qualifier) would be presuming a proposition when not only is there no evidence, but there is no warrant whatever to arrogate the presumption.
When I preach the gospel, I "presume" that my audience can hear and understand it. The believers can to begin with. And the unbelievers may, by this preaching of the gospel, be believers at the end of it. That's gospel power. That's effectual calling. A non-regenerate heart is a dead heart. It cannot respond to anything spiritual.
What about my children? I assume/presume/whatchacallit that they can hear and understand my spiritual discipline when they are 1 yrs old. I don't wait until they get converted to start this process, because "they can't be regenerated yet." I'm acting in faith that my spiritual discipline, and calling them to repentance and faith in Christ, will be the means to their conversion.
Reformed people of all stripes these days go to great lengths to deliberately avoid the word "presume", even though older, pre-Kuyper stalwarts like Charles Hodge may have used the term quite freely. Some of the anti-PR Dutch use terminology having to do with covenant-identity to say essentially what I've said here. Word-games tend to annoy me, so depending on my mood, I will just say I presume properly and not improperly, and leave it at that. My "presumption" is variable and situational, subject to change.
To address the real subject of the thread:
We never assume anyone's salvation. We believe and act based on how revelation tells us to believe and act regarding people of any kind, with the evidence they present us. The person described is a candidate for intensive pastoral and elder care. But as long as he is coming to church, presuming it is a good church, he's getting the gospel preached to him week in and week out. And that is the only way anyone ever changes. Anyone.
The young person is giving evidence of deep rebellion. But only he (maybe) and Holy Spirit know what his true heart condition is. Maybe he's never been converted. There doesn't appear to be "first signs" of conversion, but we are getting in on this "story" long after it began. Were there positive signs a year ago? Five? Ten? See, we have very incomplete knowledge. What does the boy say? What kind of claims does he make? How well has he been catechized? What is his professed hope (if any) for eternity?
At his late age, based on his lack of any spiritual interest (which I will stipulate he has NEVER shown), his love of sin, etc., I would "presume" at this point he is unregenerate and unsaved. Until I start preaching. Then I will "presume" that the Spirit is present and may be working right then on his heart, and he CAN hear from his heart, and be converted. And afterward, I will reevaluate the situation.