Why do they say the younger u are the most likely to get std?

Because it's true: The reasons are complex. 19 year olds are no more susceptible to most STDs than persons age 25 or 30. But young people tend to change partners more frequently, to have occasional high risk partners, and often less likely to use condoms consistently. Older folks often have had STDs previously and therefore are partly resistant or immune. Protect yourself: use condoms, get vaccinated against HPV.