There's a lot about Jefferson Quishpe that isn't unusual for a 13-year-old boy. He loves Dragon Ball video games. He despises vegetables. He wears his hair short and spiky. But the similarities fade on stifling hot days, when Jefferson refuses to put on a pair of shorts.

''Everybody can see my legs and I don't want them to,'' he says, staring down at his Nike P-Rod sneakers.

Crooked legs like Jefferson's haven't been seen much since the 19th century, when rickets -- the childhood disease that softened Jefferson's bones -- crippled kids working in factories. Rickets is caused by extreme vitamin D deficiency; vitamin D, necessary for the body to absorb calcium, comes primarily from the sun.

Rickets was all but wiped out in the 1930s with the advent of fortified milk and kids spending more time outside. In the last 10 years, however, a stream of reports has announced the return of rickets. Although small in number, these cases foreshadow a larger problem of vitamin D-deficient children.

The fear? Today's generation will be more vulnerable to bone-cracking osteoporosis and other serious conditions recently linked to vitamin D deficiency, from diabetes to cancer to multiple sclerosis.

''This potentially is a time bomb,'' says Dr. Laura Tosi, bone chief at the Children's National Medical Center in Washington. ``What I worry about is that an affluent country like ours is growing a population that is going to be old before its time.''

Medical experts can't pinpoint how widespread rickets has become because there is no national data on the disease. But after a review of Georgia hospital records in the late 1990s, the Centers for Disease Control and Prevention estimated five per every one million children under the age of 5 in this country is hospitalized with rickets. The actual numbers are believed to be higher because only 20 percent of the cases wind up in hospitals, says Kelley Scanlon, a CDC epidemiologist.

Those most vulnerable: dark-skinned, breast-fed infants. Dark skin has more difficulty absorbing vitamin D from the sun and human breast milk doesn't contain enough D.

It's the children whose vitamin D levels are low -- but not low enough to develop the obvious signs of rickets -- that doctors worry about most.

''Almost every single day, I see a kid with low vitamin D levels,'' said Dr. Ana Paredes, director of clinical research at Miami Children's Hospital. She recently screened children with chronic diseases like asthma and diabetes and found 25 percent had low bone mineral density.

''It's become extremely prevalent, even in the Sunshine State,'' she said. ``We've gone from being cautious about sun exposure to now nobody gets sunlight or, when we do, we use extreme protection. The other factor is kids are getting weaned off milk very early in life.''

Childhood is a critical time for bone health, as the majority of peak bone mass develops during adolescence.

''Osteoporosis is not a disease of adulthood; it's a disease of childhood,'' says Dr. James Dowd, a Michigan rheumatologist and author of The Vitamin D Cure (Wiley, $24.95). ``Think of it as a retirement savings account. You save all your bone for retirement before your 20th birthday. If you didn't save enough, you're going to run out.''

A blood test can determine vitamin D levels, but it's not something most pediatricians check.

SAW SIGNS

Jefferson's mother, Monica Chuga, says she knew something was wrong when her son was still struggling to walk at age 5. Born in Ecuador, Jefferson came to the United States when he was 6 after a group of American volunteers sponsored him to go to Tampa's Shriners Hospital.

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