Why do we sometimes witness someone (usually a young person) state that a person's age has something to do with relevance?

When someone like Jay-Z, E-40, KRS-One, Nas, etc. has an opinion on the state of hip hop, someone will usually resort to saying the person is 'old' and needs to sit down somewhere.

On the other hand, some of the so called vets will have less than flavorable comments about the young.

I think both have a say in hip hop, however, each are only stating their opinion of a matter.

Is it the Willie Lynch syndrome (if you don't know about Willie Lynch, google it)?

What causes the battle between age groups? Jealousy or a lack of understanding one another? 

i'd say not understanding and maybe jealousy.

alot of what the younger crowd is into is strange and unknown to the older crowd. especially in the case of rap where things are alot more extreme than those who are weathered from a milder upbringing. now i will say that the wilder people of from an older generation are more supportive of todays youth. some even are more hip than the youngsters. and this is when the hate from the youngsters starts to pour in.

but i see it as this. and this is just in general as an example:  when your mom and dad were young, your grandparents hated their music. your parents hate your music of your time. and you dislike the music of your childrens generations.

thats the cycle it seems to go in. but like most hate it does nothing productive for either. the old have to share their wisdom and not judge. while the young have to learn and take heed. it's the only way any will truly see any progress.