Understanding Society User Support

Hi, I'm interested in looking at people's transfers to their non-resident adult children. I'm wondering if it is possible to create an accurate variable in BHPS 18 which indicates whether a respondent has any non-resident children who are aged 18 or over. As far as I can see, the relevant variables are:

Whether the respondent is natural parent of any children (lprnt in xwavedat)
number of natural born children (lnprnt in xwavedat)
The age of the 1st born child (ch1by in xwavedat)
The number of children still resident, and their ages (derived from the household grid)

My two problems are: 1) Without knowing exactly how old each non-resident child is, I can't see how to create an exact indicator, 2)When I merge lprnt in from xwavedat, there are 7,343 cases marked 'not applicable'. From the documentation this would suggest that they were never asked whether/how many children they were a natural parent to. Is this the case? If so, is there any way of recovering the information I need?

Best,
Rob