What age-cheat would be so moronic as to pick an age that already made them too old for their grade?
1) Lithuania has a well-established health-care system, and since Verzbicas came from a middle-class family, it's very likely that he has an official birth certificate.
2) He arrived in the country when he was ~9 years old. He also had very limited English, and based on his Lithuanian education up until that point, he was held back one year.
3) By high school, his English had substantially improved, and provided his school district is to be trusted, he took additional courses in order to catch himself up and graduate after his junior year.
The biggest factor that clears Verzbicas from being an age-cheat though, is the fact that he has been in America too long. Let's just take your original statement for a moment, and assume that Verzbicas was 25 as a high school junior, which is 7 years older than he was reported to be. This means he would have had to have been 23 during his freshman year, 22 as an 8th grader, 18 as a 4th grader, and 16 as a 2nd grader.
Do you honestly think that a grade school in suburban Illinois wouldn't have noticed something strange happening when a 16 year old showed up claiming to be 9?
Seriously, go watch a video of one of his races during his freshman year. He was tall, lanky, and looked like a child.
Here's an end-race photo:
http://www.nationalscholastic.org/article/1263And here's a pic of him in a race with two normal 18 year olds, Jantzen Oshier and Elias Gedyon:
http://www.milesplit.com/meets/88735-adidas-grand-prix/articlesLook at his face compared to Oshier in front of him, and his arms compared to Gedyon behind him. If he was 25 during this race, he's by far the most underdeveloped 25-year old to ever break 4:00.