Actually Trump is about 6 months older than Ronnie when he became President. Not that it means a hell of a lot............Ronnie went to 93.
He was utterly gaga ... a vegetable at the end. Now George Bush Senior is all there, something for Junior to aspire to.
It's doubtful. Mexico isn't much of a threat. I'd be more concerned with the drug cartels. The Mexican police have a rotten track record dealing with them. The drug lords get the upper hand more often than not.
WRT the US, Trump is getting more border police, but I don't expect the Federales to be marching across the border any time soon.
If the Mexicans fight back, the Americans immediately enter the history books as a "rogue nation". The Mexicans could probably put together a cracker of a military alliance (China? Iran? most of South America? Vietnam?) in a hurry, if they do choose as the Americans have LOTS of enemies out there. If America starts a war with a peaceful neighbour based on a whim, they might be seen as another NAZI Germany that has to be stopped. Fiction . Maybe.
Pissing off peaceful neighbours because they aren't white enough isn't the brightest move, at the least.