There seems to be several groups out there in which one or more members are at least 5 years older than the other band members.

For example:
The Rolling Stones
AGE GAP: From 1962-93, bassist Bill Wyman was the oldest member (born October 24, 1936), whereas the youngest members of the band were Mick Jagger and Keith Richards (both born 1943, on July 26 and December 18, respectively). The age gap widened from 7 years to 13 when Mick Taylor (born January 17, 1949) replaced Brian Jones (born February 28, 1942) in 1969, but was reduced by 2 years in 1974-76, when Ronnie Wood (born June 1, 1947) replaced Taylor. When Bill Wyman departed in early 1993, drummer Charlie Watts (born June 2, 1941) became the oldest Stone, reducing the age gap from 11 years to 6.

Even keyboardist and road manager Ian Stewart (born July 18, 1938) is a good 5 years older than Jagger and Richards.

The Police
AGE GAP: Andy Summers (born New Year's Eve, 1942) is nearly a decade older than Sting (born October 2, 1951) and Stewart Copeland (born July 16, 1952). The previous guitarist, Henry Padovani, is actually the same age as Copeland, just three months younger (born October 13, 1952).

'N Sync
AGE GAP: Chris Kirkpatrick (born October 17, 1971) is nearly a decade older than the group's youngest member, Justin Timberlake (born January 31, 1981).

The Go-Go's
AGE GAP: Guitarist and keyboardist Charlotte Caffey (born October 21, 1953) is nearly 6 years older than bassist Kathy Valentine (born January 7, 1959).

The Who
AGE GAP: During the Detours years, a drummer named Doug Sandom (born 1936), who is 9 years older than guitarist/leader Pete Townshend (born May 19, 1945).

Fascinating..

The Wallflowers is one good example. There is an age gap between two former members from the Bringing Down the Horse and Breach years, guitarist Michael Ward (born February 21, 1967) and drummer Mario Calire (born June 25, 1974). To be exact, Calire (who left the band in 2003 and is now the drummer of the experimental musical group Ozomatili; he was replaced by official band member Fred Eltringham) is younger than Ward (who left in 2001 when the group was recording their fourth album Red Letter Days; he is now somewhat irreplaceable to the band and unofficial member Stuart Mathis now takes his place) by seven years.

I also want to add:

Supertramp
Founding singer/keyboardist Rick Davies (born July 22, 1944) is six years older than guitarist/keyboardist/singer Roger Hodgson (born March 21, 1950), a six-year age gap between oldest and youngest for the most well-known lineup from 1974-83.

Update on the above post on the Wallflowers since I've written it: Stuart Mathis is now an official member of the band and Jack Irons has replaced Fred Eltringham.