You've gotten many good answers here about neighborhoods.....but I would just like to ad, that when you are working with a realtor, they have the ability to screen for 55+ neighborhoods. Most of the time, it is unlawful to steer or discriminate for any reason, however there is housing that at least ONE of the prospective residents is at least 55. If this applies to you, then dont forget that a realtor can screen for and provide you a list of available homes in these communities.
Jensen White Real Estate
Vancouver WA (just across the Columbia River to the north of Portland)
Other areas that are quite a bit more affordable but quite a bit further from downtown Portland would be the towns of Hillsboro, OR, Gladstone, OR or Camas, WA. They also have the combination of being fairly quiet communities with good neighborhood vibes.
There are many more neighborhood descriptions on my website, and I'd be happy to help you if you are considering moving to Portland.
Oregon First, Realtors
Tom Inglesby, Broker
RE/MAX Equity Group Inc.
John L Scott PTL