Their love is certainly believable to older people because most  older men and women have been through the same experience themselves. If their love isn't "real" love, then what is real love? Neither Romeo nor Juliet stops to ask, "Is this really love?" Shakespeare was writing a poetic tribute to the essence of young love, with all its emotions and all its mistakes and heartaches and obstacles. If Romeo gives himself heart and soul to beautiful young Juliet, and if Juliet gives herself heart and soul to handsome young Romeo, isn't that true love? Only young lovers--unfortunately--are able to give themselves to each other with such abandon. When people get older they have become a little gun shy, a little more practical, a little more selfish. Also, they want to go to bed at a reasonable hour and not be climbing over walls in the middle of the night or fighting duels with their loved one's relatives. They are both impetuous. They both make terrible mistakes. They are young, but they are both so enviably energetic, daring, passionate, and headstrong, and unselfish and foolish and impractical! Tchaikovsky wrote a beautiful tone poem titled  Romeo and Juliet  which says in music what Shakespeare is saying in poetry.

To be young. To be young. There is nothing else like it: there is nothing else in the world. -- William Faulkner, Light in August