It should be translated as 'they have been here for ages'. Similarly:
dyn ni'n byw yma ers achau. - we have lived here for ages
The use of the present tense in the Welsh is correct in this pattern, but not in the English!
Yes fine, but in another sentence, 'for a while' surely implies a shorter time and 'were here for a while' for 'yma ers sbel' seems a valid translation
See the course notes.
In this pattern, where Welsh uses the present tense (Maen nhw yma ers...), it is translated into English as the present perfect tense (They have been here for/since...).
This is another example of where English and Welsh structures are quite different.