I think it's a great phrase, and sadly, it's looked down upon. It has nothing to do with "disrespect the young", no one says "give the young less love",
I disagree. Its roots are, imo, very much rooted in the mindset of "children should be seen and not heard", which is one that's very disrespectful of children.
but there IS a reason why so many cultures give the elders a special place in the society. I love my grandmother and I love my mom. I have a complete fascination with how much my mom has lived through, and the kinds of experiences and difficulties and wisdom she has gained in her lifetime, however, it's almost twice as much for my grandmother! My respecting what my grandmother has been through has NOTHING to do with me loving my children any less, or abusing them, or mistreating them, or insisting that they feel worthless when talking to grandparents.
This is interesting, and it's worth thinking about, but it doesn't always apply. I know both my mom's and maternal grandmother's life stories in a fair bit of detail. My mom had been through more in her life by the time she was 25 than my grandmother went through by the time she was 60. My mom also learned from her experiences, which my grandmother didn't. My father-in-law has been through an amazing amount in his life, and I have a lot of respect for many of his insights and observations...but he's learned from those experiences, as well. Different people show different levels of being able to learn from their (or other people's, for that matter) mistakes, and being old doesn't mean someone has necessarily learned a lot. Years of life aren't like blocks or pennies - one isn't always equal to another.
My grandmother has been through all the life stages that most of my family is yet to experience and that deserves some recognition. I find it incredibly sad to see that some young kids treat older generation with smirks, pity, or disgust. It's as if they believe they will never be old themselves. I think that's what the phrase is meant to address.
I don't believe that's what this phrase is intended to address, and I don't think I've ever seen or heard it being used in that fashion. Young people who are treating the older generations with "smirks, pity or disgust" need a lesson in basic respect for people, not for elders.