As I understand Price's plan:
1. Yes, credits are age based. $3000 for 50+ though isn't a lot. His plan does not cap at all the premiums that can be charged. Currently, I think older people can only be charged 3x what younger one's are charged. Some proposals have suggested a 5x rate. Price's plan puts no limit on what can be charged.
2. His proposal on pre-existing conditions allows people to be covered at standard rates if they maintain continuous coverage. If there is a gap in coverage (example: person loses job and can't afford coverage for awhile) then they have to pay 150% of standard rate. They get back to standard rate after 18 months of continuous coverage. That is actually not that bad, but the real problem is that cost of coverage for older people is likely to be very high.
For example, I read an article where someone commented that on the marketplace in their locale the cheapest bronze plan for someone over 50 was about $8500 a year. I get private retiree insurance which is partially subsidized by DH's former employer. It covers me and our 2 young adult children -- the premium is over $18000 a year, most of which is for me.
(Note that $8500 premium for similar coverage would likely be far more expensive under Price's plan because now insurers are constrained to 3 times what young people pay. Under his plan, the sky is the limit).
3. There are no mandated requirements for what has to be covered. That can potentially result in lower premiums for policies that cover less. On the other hand, there may not be policies available with the coverage you want (or they may be priced extremely high).
4. Medicaid expansion is repealed without any replacement for it.
5. The employer tax exclusion for employer provided plans is lowered.
The overall gist of his plan is that it may save money for young, healthy people who like plans with lesser coverage and lower premiums.
For older people who may have significant health care needs, premiums would be unregulated with only a $3000 credit provided. For someone in that age bracket, I think it would significantly increase my health insurance budget.
6. Note that going on Medicare may not help later on. Price is an advocate of privatizing Medicare and providing only a voucher that would cover part of the cost. He has indicated that Congress will get to work on the Medicare part in about 6 months.
