A paper from the National Bureau of Economic Research looks at the long term effects of removing young people from SSI payments at age 18. The consequences appear dire. According to the authors:
We find that SSI removal increases the number of criminal charges by a statistically significant 20% over the next two decades
In response to SSI removal, youth are twice as likely to be charged with an illicit income-generating offense than they are to maintain steady employment at $15,000/year in the labor market. As a result of these charges, the annual likelihood of incarceration increases by a statistically significant 60% in the two decades following SSI removal.
The costs to taxpayers of enforcement and incarceration from SSI removal are so high that they nearly eliminate the savings to taxpayers from reduced SSI benefits.