In Spanish, the word for ‘old’ is viejo when referring to masculine nouns and vieja for feminine nouns. For example, you would say el libro viejo (the old book) for a masculine noun and la casa vieja (the old house) for a feminine noun.
It’s important to note that context matters in how you use this word. While viejo can simply mean ‘old’, it can also carry connotations regarding value or respect based on the context in which it’s used. Additionally, in more formal contexts or when speaking with respect, you might hear the term de edad avanzada, which translates to ‘of advanced age.’