While you cite Ramsey, Revie and Greenwood for not giving debuts to 18 year olds, that doesn't mean the rest of the world weren't, for example
The real difference between then and now is that it's far more likely for a player to change the nation they represent, for example in the 60s the Portuguese were able to stick Eusebio on a plane in Mozambique and chuck him on a pitch in Lisbon to guarantee he was a Portuguese international, but that's not the case these days as players can switch allegiances as demonstrated with the likes of Declan Rice, Jack Grealish, Wilfried Zaha, Victor Moses or Dom Dwyer to name but four that means that if an FA keeps a player waiting too long they can up and leave for another, or in the case of Rice and Grealish basically become plastic internationals (which hasn't exactly worked out for Grealish, has it?) while Dwyer was never going to play for England so a call from the USIEAIAIO was a no-brainer
That is a good point and to be fair of the 10 who have debuted under 18 since Owen - (Walcott, Rooney, Sterling, Owen, Richards, Sancho, Rashford, Wilshere, Shaw, & Oxlade Chamberlain) - 2 are definite successes, 3 are doing well and likely to be judged a success, 2 the jury is out on and 3 have had what i would call indifferent international careers

