When I was a young coffee house singer, I would greatly admire those with a vocal resonance and vibrato I could not command. Over the years, I heard many vocalists try too hard to "create" vibrato where none existed. Kenny Rogers was one who made it very obvious at times. It wasn't until I hit my late thirties and forties that I learned to "relax into it." I think younger people are often more tense, voices are somewhat more taut and often a bit higher in pitch as a result. As one ages, the voice deepens and acquires more timbre and color. Maria Muldaur once referred to her young voice as like a clarinet that deepened and became colored more like a tenor saxophone as she matured. It is at this point that vibrato seems to occur most naturally. For me, it was learning to relax and to breath from the diaphragm instead of the top of the lungs. Frank Sinatra said he learned his phrasing and breathing technique from brass and woodwind players. Not a bad place to begin.