In BrE, we don't really use the phrase "senior citizens". I think that's more AmE. We refer to those over retirement age as OAPs, pensioners, the elderly. You might see "seniors" too.
In the UK, there are lots of different terms for communal homes for older people, depending on what services they provide. We have old people's homes, care homes, nursing homes, retirement communities, warden-assisted housing and EMI homes (EMI = Elderly Mentally Infirm).
The plural is always used when talking generically but if you are talking about a specific one, then it would be a nursing home.