Getting rid of SS is a death by a thousand cuts situation, since Republicans know they can't do away with such a popular program by enacting big changes, so they propose seemingly minor changes, such as these, couched in misleading language intended to make people think that this will be good for low-income folks at the expense of high-income folks. This is misdirection though, and it's not conjecture to assume as such, since it is perfectly ideologically consistent with them wanting to get rid of SS altogether, a goal that Ryan and other Republicans have been very clear about for forever.
As far as these specific cuts: raising the age for receipt of full benefits is a back door way to cut benefits across the board because most people have the take the reduced amount at an earlier age. Few people are able to work full-time until 69: due to physical reasons, for laborers, and because they are pushed out, for professionals (we don't live in a culture that particularly values experience and wisdom). This is I think one of the bigger "cuts" in the plan since it affects high- and low-income workers alike.
As outoftime mentioned, this plan is bad for people who don't have consistent full-time work histories. Women (mostly) who take time off to be home with young children, people who take time off to care for ailing/dying relatives, those with chronic illnesses, anyone who experiences a long spell of unemployment, and early retirees will receive reduced benefits, regardless of peak earnings, due to their shortened work histories.
Cutting benefits for 15-18 year olds who have graduated will hardly save any money whatsoever, but why pass up an opportunity to limit the benefits of teenagers with deceased parents? Added bonus: 15 to 17-year-olds can't even vote, so it's not as if this is a constituency they need to answer to. Also note, that this cut will add an additional layer of bureaucracy to the system, since teenagers receiving benefits will have to go through an extra step to prove eligibility, and that bureaucracy doesn't pay for itself so it will certainly cost money.