You can use the word either like an adverb or as an adjective.
If the word is tacked directly to a noun, it's an adjective and gets flexed according to the gender, case and number the noun is in:
nominative: eine alte Lampe
genitive: einer alten Lampe
dative: einer alten Lampe
accussative: eine alte Lampe
But you can also use the word as an adverb:
Die Lampe ist alt (the lamp is old).
In this use, the word will not get any declension.