You're in the WoF forum, so I'll answer according to common WoF doctrine. Preaching is actually a form of prophecy and an evangelist must be able to preach and have the working of miracles, gifts of healings and often the gift of special faith. Smith Wigglesworth had all these in operation, that's why he was able to raise the dead. Most so called evangelists are actually exhorters and are NOT true evangelists. Again, working of miracles SHOULD be evident in a TRUE WoF evangelist. Billy Graham, according to WoF, would be considered an exhorter. If you want a different answer go to the Baptists.

ETA, 50 is not too old. I believe Smith Wigglesworth started preaching at 57 and had an outstanding ministry with numerous signs and wonders. If God has called you, He will equip, no matter what age. Go for it!