Hmmn
Over the last 250,000 years/several million, that humans have been human nobody would have batted an eye over this.
This sort of concern is really a very recent thing,
Really very recent, only a couple of generations (If that).
So, (Controversial question) What is the problem?
Not sure that's a great argument...... At various places and points in time adults having sex with either
relatives or with
children (by the modern legal definition) wasn't seen as a big thing or was actively endorsed.....
Even in the UK in 2018 you can discern which particular populations have far higher cultural acceptance for marrying their relatives by their drastically elevated levels of congenital defects compared to others.
Slavery didn't seem so out of the ordinary as well for most of that time as it was a quite common practice widely utilised.
Most of that time, by contemporary standards, was quite often extremely brutal with humans having a far lower life expectancy and with a much increased risk of death due to pregnancy.
So it made some sense to have (many) children when women were younger *(when the risk of complications was sometimes lower) and so the mother was likely to be around a while with her offspring....
*(although being a very young mother drastically increases the risk of death vs being in your later teens or early twenties)
The avergage age at which a woman becomes biologically 'fertile' did not develop in the human species driven by the welfare considerations of the individual women concerned but as a response to evolutionary pressure which was just directed towards preserving the species as a whole....
So while a 14 year old female may have devolped biological maturity sex wise its recognised that they may not be best placed mentally or societally to deal with sex and its likely outcomes.
Old men marrying rather younger females was quite common practice in places like Europe to the middle East and beyond (cue now potentially awkward stories about a 53 year old prophet and his 6 or maybe 7 year old bride) ... Should this still be considered acceptable?
In answer to the Op post..... it's mostly pointless to try and stop this relationship just make it clear what the law says about the age of consent and point out that the boyfriend can find himself a sex offender on a register if he makes a bad decision.....hopefully your daughter has aspirations of makimg something of her life and realises that its best to delay the possibility of childbirth at 14 and has the self confidence not to go along with something out of pressure be it peer or any other sort.