Reference Links: Half-Elves Confirmation that Elves did in fact evolve from trolls and that trolls were in Kalimdor way before the third war, thus debunking the statement that trolls couldn't have mated with elves before the third war and could only be children at the present time. Source 1 + Source 2
Additional info
First - Gates of Ahn'Quiraj
Second - War in Outland
Third - War in Northrend
Fourth - The Cataclysm
Reference Links:
Half-Elves
Confirmation that Elves did in fact evolve from trolls and that trolls were in Kalimdor way before the third war, thus debunking the statement that trolls couldn't have mated with elves before the third war and could only be children at the present time. Source 1 + Source 2