Gymnasts Nastia Liukin and Shawn Johnson were precocious teen prodigies when they co-starred in the 2008 Olympics. Combining for nine medals would seem to have put them in good stead for London, but the career clock ticks extra-fast in this sport.

Liukin, 22, took two years off, hoping to recharge the batteries. However, the defending all-around champion, who needed an extra carry-on bag to bring home five medals from Beijing, has not revved back up to full voltage entering this week’s U.S. Trials in San Jose, Calif.

At least the Russian-born Liukin will answer the bell. Johnson, 20, was forced into retirement by an injured knee. She will console herself with four medals from Beijing, notably a gold and the all-around silver.

A bevy of teeny-boppers should keep the bar — parallel, horizontal, uneven — high for America.

At age 16, Jordyn Weiber already has accumulated enough gold — world title, two national crowns and plenty more — to open a bank. She will feel the heat from other sweet-16s-and-under such as Gabby Douglas, plus veterans Alexandra Raisman, Bridget Sloan, Chellsie Memmel and two others bouncing back from injury, Rebecca Bross and Alicia Sacramone.

Then there is McKayla Maroney, whose participation is iffy because of a mild concussion suffered at a competition. Maroney must take the stage in San Jose to remain eligible for the team. Unlike the guys, the girls cannot petition for a roster spot based on an injury absence.

The U.S. men are catching up to their leotarded teammates. Beijing standout Jonathan Horton hopes for an encore, while Danell Leyva, John Orozco, Steven Legendre, Chris Brooks and Sam Mikulak are in the medals mix.

Improved depth on both sides alone guarantees oodles of drama at the Trials. Ratcheting it up further is a change that limits each nation’s Olympics squad to five per side, down from six, to create more slots for developing gymnastics nations.

So, beware, established stars. Everyone else: Look out for falling stars.