For me it was the trench art made by my Grandfather in WW1, Canadian28th Infantry Division veteran.
Interest already peaked, a healthy dose of Warlord and Battle Action comics in the 1970's sealed my fate and led to a deeper interest in both World Wars and the start of my Militaria collection in my teens.
That waned in my 20's and re-started in my 30's as I settled on collecting and shooting Britishand US Milsurp firearms. Now in my mid 50's and still enjoying it....