It has some advantages over more traditional welfare systems. For instance it could be implemented with far less bureaucracy and would help improve labor mobility in the middle class by decreasing the friction of changing jobs and moving to different areas with better jobs (improving labor mobility would benefit the economy at large).
18 is probably too old. We want the child support handout to be separated from the parent as early as possible. Giving 12 year olds their own income is probably better than either welfare queens or fertility police.
It has some advantages over more traditional welfare systems. For instance it could be implemented with far less bureaucracy and would help improve labor mobility in the middle class by decreasing the friction of changing jobs and moving to different areas with better jobs (improving labor mobility would benefit the economy at large).