i'm younger than my closest sib by 6 years, dh to his closest by closer to 10. so i'm posting from 'the baby's' perspective.
no matter what you do, that age difference, at different points in their lives will make for them to be experiencing and enjoying things differently such that you can't please both at the same time.
based on that-if you have the family dynamic that people do things just because it's enjoyable for someone other than themselves-that's fine. if not-don't expect a teen to enjoy a geared to young child's bday party or outing, and don't expect a young child to behave 'appropriatly' at a teen oriented event (it's fine to leave the little one at home for an older child's special school or sports events-the teen would prefer you focus your attention on them or better yet not draw embarrassing attention to them by virtue of their younger sib there and acting inappropriatly-and the younger sib could'nt give a hoot about being there unless you've inflated 'how fun it will be' in their mind).
i don't think parents give enough credit to 'the babies' of families with older sibs. they don't realize that they are well aware when their teen sibs are being forced to spend time with them or have them included or go to bday type parties they would'nt otherwise be caught dead at. the younger sib does'nt nesc. view it as 'he loves me so much he's coming to something he hates' but can rather view it as 'he's only coming because mom will get mad/won't let him do.../told him he loses his priveledge to...if he does'nt come and pretend to enjoy it'.
that said-plan the bday parties as you would for any child-not based on what one or two of the guests (as in the teen sibs) would want, but what the majority of same age guests (as the bday kid) would want (or the bday child wants). don't try to force a relationship-they are at very different stages in their lives-2 are entering adulthood, one is well entrenched in childhood (and believe me, it gets harder when the older two become full fledged adults, perhaps have children and begin (i think out of habit with their own kids) to exert a parental authority on what will then be your young teenager).
my sibs are almost 17, 15 and 6 years older than i.
one thing i've come to realize is that while we were raised by the same parents, i was raised by very different parents than my sibs. my parents were older by the time i came along-so my sibs would say that my parents were "broken in"
and maybe a bit more indulgent with me (i would dispute that-my sibs taught them all the tricks so they were more wary with me). i would counter that i was the one, during my teen years, at home overhearing (despite my parents best efforts to shield me) their discussions/concerns about my young adult sib's behaviour/college issues/relationship issues/marital issues. i was the child that was with mom and dad during an entirely different period of their lives. it was'nt nesc. better or worse-just different, so my attitudes and habits are different-we have very different perspectives on what our family lives were like.
i don't mean to scare you, just to make you aware-in the long run it will be the 5 year old that remembers the bday he/she was dying to go to chuck-e-cheese and could'nt because his/her older sib did'nt want to, whereas the older sib won't give a second thought about you making this accommodation to suit them.
