Those were the days, already?

Nostalgia is a funny thing. It usually takes 20 years before a trend resurfaces and is re-embraced by the masses.

For instance, the sounds and looks of the ’50s came back during the ’70s, when Happy Days was a top-rated series and Little Richard and Jerry Lee Lewis were back on the circuit. Flower Power and rebellious icons from that era were celebrated during the Reagan administration. It seemed as if everyone had a copy of No One Here Gets Out Alive, the Jim Morrison biography. Even Gary Puckett and the Union Gap were getting gigs again.

Who would have thought disco would make a comeback, but it did just that during the ’90s. One-hit wonders such as The Trammps and bona fide talents like Chic once again filled the airwaves.

So it’s hardly a shock that such hair-metal bands as Poison and Ratt are back playing 3,000-capacity venues these days. The former recently headlined a show at the Tower and the latter was on a bill at the Electric Factory. The B-52s and Blondie also are on tour.

It’s time for these bands to return, as studies have proven that for most people, the music you’ve been exposed to during your college-age years becomes your favorite music for life. Twenty years later you have some disposable bucks, but you probably have a family and are so busy working that you could be divorced from what’s contemporary. It’s just so easy to slip back into what’s comfortable, what you listened to back when you had more hair and fewer wrinkles.

What’s interesting is the return of the early ’90s — a tour of which is coming to a venue near you by year’s end. Yes, it’s 10 years too soon. But tell that to the bands on the New World Disorder tour. The Gin Blossoms, Spin Doctors, Sponge and Seven Mary Three are planning a jaunt that will bring the bands back to amphitheaters as if it were 1992 all over again.

But it’s not ’92 — and it won’t be for another decade. Perhaps some of the members of these bands took a big hit with their stocks and have to get back on the road.

I have to admit I have a soft spot for the Gin Blossoms. The Southwestern band had a knack for writing simple, pleasant, catchy pop. The quintet crafted solid, memorable love songs, which are in short supply on pop radio these days. The group, which splintered during the late ’90s, never said its split was final, but it seemed that way. The Gins now plan to record a new album.

Last September, the Spin Doctors returned to the stage to close New York’s Wetlands, where their career took off during the late ’80s. The band, which had one strong album, Pocket Full of Kryptonite, is back with all of its original members. Vocalist Chris Barron, whose vocal chords were mysteriously paralyzed, is healed. Barron has reconnected with guitarist Eric Schenkman, bassist Mark White and drummer Aaron Comess.

Sponge was a gritty, hardscrabble, guitar-driven band that made potent rock ‘n’ roll. The Detroit group never embarrassed Motown. The chords and energy of Wax Ecstatic make it one of those powerful rockers you can play repeatedly. Molly (Sixteen Candles) is a cute, buoyant pop number.

Seven Mary Three is one of those forgettable one-hit wonders (Cumbersome). The Southern band was execrable and doubtfully has improved with age.