Flora, that particular poem (
Rabbi Ben Ezra by Robert Browning) is dramatic monologue. It's about an elderly man (Rabbi Ben Ezra) counsel(l)ing a younger and less experienced man on the meaning of life. Youth may appear or seem to the be-all-and-end-all of one's existence, but it's not, suggests Rabbi Ben Ezra. Rather, Youth prepares us for our future, so take heed.
A1: The best (part of your life) is yet to come. That time when you can draw from and utilize the knowledge from the experiences you've learned.
A2: The first (part of your life) - your youth. The last (part) of (your) life
- your adulthood and senior years.
A3. "Our times =" our days, years, our life's span.
A4. "saith" is taken from the word of God; e.g., the Bible. Note that, Ben Ezra is a Rabbi. He's quoting from a religious text.
A5. "A whole I planned" = God has planned a complete life for us. In other words, say, first we go to school, then we practise what we know. School prepares us for our career in the same way that Youth is suppose to prepare us for our future. School and Youth are (the first) part of the whole. "the whole" is the complete cycle of learning. One must use one's time as a youth wisely because its purpose is to prepare one for the future. That is Rabbi Ben Ezra's message to the young man he's counsel(l)ing.
A6. "youth shows but half" (See A5)
A7. "see all, nor be afraid!" means, look at the bigger picture. The end of youth is not the end of one's life -rather, it's just the beginning. It's the portion of our life that prepares us for our future. So, don't be afraid of getting old or of old age. Look forward to it! Most importantly, prepare for it. Now is the time.