If they qualify for a visitor visa, they can apply. The main thing they have to prove is ties to their country and that they will return. People on visitor visas cannot 'live' in the UK.
If they have never been to the UK, they will only get a 6 month validity visa.
For your parents to settle here under UK Immigration rules is virtually impossible. The rules changed in July 2012 and since then, we have only heard of 1 successful application on the forum. Even with the old rules, your parents would not have qualified.
https://www.gov.uk/join-family-in-uk/eligibility
Coming to be cared for - you’re an adult dependent relative
You must be dependent on a parent, grandchild, brother, sister, son or daughter of someone living permanently in the UK.
You must prove that:
you need long-term care to do everyday personal and household tasks
the care you need is not available or affordable in the country you live in
the person you’ll be joining in the UK will be able to support, accommodate and care for you without claiming public funds for at least 5 years
you’re 18 or over
And this link, page 38 onwards :
https://www.gov.uk/government/uploads/s ... dix_FM.pdf
If you search the forum for 'adult dependent' or 'older dependent relatives' etc, you will see that there are many refusals for adult dependent relatives. Once refused for a settlement application, it is unlikely that they will be granted a visitor visa as their intention to settle and stay in the UK will be known.