Translation:The clothes of the grandparents are old.
”bunicilor” means also of/to/by grandfather, grandmothers, not only grandparents. In Romanian we have:
grandmother: o bunică, două bunici, bunica mea, bunicile mele, bunicii mele, bunicilor mele
grandfather: un bunic, bunicul meu, bunicului meu,
but the plural male forms, doi bunici, bunicii mei, bunicilor mei, etc, refers to either grandfathers, or grandparents (both male and female together).
Therefore here the translations ”The clothes of the grandparents/grandmothers/grandfathers are old” are all right, and have to be accepted.
Interesting fact, the word ”bunic” is a 'forced' diminutive of ”bun” (same way as lapte-lăptic) which means ”good”, or here, ”kind”, and its etymology shows the fact that the grandparents are always kinder to the kids than the parents are. Some regions of the country still keep the words/regionalisms ”bun” (good/kind) or ”bunel” (another way of making diminutives), with the same meaning, ”grandfather”