Did my first 14er at age 51 last summer - East ridge (class 1) of Quandary Peak in July. Would recommend it to others as a first as well. As implied, this was definitely a "long walk" vs technical, but still not a cakewalk. Def spend a few days at higher altitudes before starting. We stayed in Breckenridge (9k') which helped acclimate. And cardio training prior is a must, esp if you live at much lower elevations like me.

Everything @hikermor said is good, and I'd add that you need to be educated on the mountain overall, your planned route and the season you'll be trekking in. Maybe start-off on a few class-1's  (Quandary, Greys/Torreys) before launching on something more challenging/technical. I found the website 14ers.com to be a great resource with route info and reasonably current condition updates from other climbers.