Good questions, Rose.
I suppose most 'elderly' people have something to teach us...for example, recycling, which they learned to do during wartime. I won't go so far as to say they all or most of them have great insight or advice, but I do think it is a good thing for the generations to co-mingle. Grandparents often have more time and inclination to spend with little kids and tell stories or teach little lessons etc.
I can say I learned social grace and tact from my paternal grandmother.
I can't make a guess at any particular age for someone to be considered elderly - perhaps 80 years old as a benchmark. My mom is 76, and has a lot of vitality and probably many more years to live life to the fullest.