At that age, I'd contend that that's when a player might be most vulnerable to offerings from self-interested associations with pulling power. The introduction of the right to switch once was to protect players from this power-imbalance to the later detriment of their careers. Tim Cahill, who became tied to Samoa after playing for their under-20 team at the age of 14, is an extreme example - he saw the invite to play for them as an opportunity to go on holiday to see his ill grandmother with expenses paid - but the present rule is to protect against that sort of thing later hampering a player's international career.
In many cases someone's purest thoughts about who they are won't be singular; it can be plural and complicated. Someone might (completely legitimately) feel as equally English as they do Irish at the age of 16. That's not impurity. It's just complexity that Breen doesn't seem prepared to acknowledge. I would suggest it's easy for Breen to talk as, even though he was born in London himself, his father was from Kerry and his mother was from Clare. I think that's pretty evident.
His thoughts on residency are also interesting. The stipulation at present is five years and possession of nationality. (Rugby and cricket eligibility rules are incredibly lax in comparison, to the best of my knowledge, so hardly worth mentioning in this context.) Breen thinks of taking on a new nationality as just "living in the area", but if a country deems an individual a citizen (thereby allowing him or her to enjoy all the civil rights that come with that) and they've contributed to life in that country for the requisite period of time (five years is a considerable period of time), why shouldn't they be entitled to be selected for the national football team? It won't necessarily be a career-move; some might see it as a means of returning something to their adopted country. It was interesting that before Diego Costa was declared eligible to represent the Spanish national football team, he would actually have been entitled to represent his adopted country in the European parliament.