I was asked "What age range is best for siblings to play and interact with one another?"
Playing is simply engaging in an activity for enjoyment.
My seven children range in age from 26 to 8. Even though he has his own house, my 26 year old will sit and play Legos, play in the pool, throw water balloons with, race Matchbox cars, etc for hours now and then with my 8 and 10 year olds. He often invites all six of his siblings to his house for pizza, movies, games, and sleep overs. They laugh and have a blast together.
When I have the time for it, I am out there with them playing just as hard. I am 48, almost 49. When I play with a baby's feet and make him laugh, I am not counting the age difference. I am counting the smiles we both get out of the shared fun experience.
It's not the age, but the love in the relationship that keeps people of all ages interacting and relating to one another. It's up to the parents to teach their children to value and preserve relationships.
I told my older children it was up to them to reach out and build those relationships with their youngest siblings. They have taken that advice and run with it.
My oldest daughter is 22, and she plans "Girl Days" where the four girls go shopping, go out to eat (her treat), do a craft, or do hair and make up while watching a movie. She and my two youngest went paint-balling with her co-workers recently.
Like my 96 year old grandfather who kept a collection of modern toys that he showed off simply because they delighted him, I intend to keep engaging in fun (playing) until I die.