It is an issue (as a doctor who worked in the hospital next to Heathrow I have met “children” who were clearly older than me, and who had been “tortured” by having a fistula created to allow dialysis == health tourist). So I’m not denying that there is deliberate fraud. There will be fraud in any system if there’s an incentive to lie, as there is in asylum cases.
But I have also met patients who were here perfectly legally who had no idea how old they were. Not everywhere registers births, not everywhere keeps track of precise ages. Some rural societies just class people by generation, so child, adult (from about 13-14), and old (from about 40). So from their point of view it made no difference whether they were 17 or 30, or if they were 50 or 70.
It works both ways, I have met supposedly 80yr old Bengali grandmothers who had children who were in their late 20s and who weren’t yet born during Partition. They weren’t deliberately lying, they just had no idea how old they were so picked a number that sounded right to them. And they had been venerable old ladies in their society, not fit working women in their early fifties, so they picked a number accordingly.
And the “sending boys ahead” - have we forgotten wartime evacuees and the Kindertransport? You may have too many ties to leave a war zone yourself (you risk losing your land and livelihood) but if you can get your children to safety you would send them. Pre-pubescent children are obviously too young to travel unaccompanied. Girls in their teens are probably married off already so no longer your responsibility. But teenage boys are seen as young adults, so probably the safest to go. And they can work outside the home, which girls often can’t, so can hopefully support themselves.
Boys are also likely to be in the most danger if they stay - invading armies will treat all young men as potential combatants, whereas old men, women and children may be sexually abused or robbed but hopefully won’t be executed or conscripted. If you look at the Yugoslavian mass graves, a lot of the victims were young civilian men who were rounded up and shot. Not always of course, some massacres murdered whole communities, but it’s common enough to be a legitimate concern if you’re the mother of a fifteen year old boy in a war zone.