alt
old
Having reached an advanced age or having existed for a long time.
Dieses Schloss ist über 500 Jahre alt.
This castle is over 500 years old.

old
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old
Having reached an advanced age or having existed for a long time.
Dieses Schloss ist über 500 Jahre alt.
This castle is over 500 years old.

When forming comparisons with 'alt' (old), the vowel changes to an umlaut: 'alt, älter, am ältesten' (old, older, oldest). This change is important for correct comparative sentences.
Mein Bruder ist älter als ich.
My brother is older than me.
The exact meaning of 'alt' depends strongly on the noun. 'Altes Brot' (stale bread) is no longer fresh, an 'altes Auto' (old car) is used, and an 'altes Schloss' (old castle) has a long history.
Dieses Brot ist leider schon alt und hart.
Unfortunately, this bread is already stale and hard.
To state the age of people or things, you use the fixed structure: subject + 'sein' (to be) + [number] 'Jahre' (years) + 'alt' (old).
Mein Großvater ist neunzig Jahre alt geworden.
My grandfather turned ninety years old.
In the meaning of 'ehemalig' (former), 'alt' almost always stands directly before the noun. It describes something that belonged to an earlier phase of life.
Ich habe am Wochenende einen alten Schulfreund getroffen.
I met an old school friend at the weekend.
In the combination 'alter/alte + noun', 'alt' can express affection and a long acquaintance. It is a very informal and friendly expression.
Na, du alte Schlafmütze, bist du endlich wach?
Well, you old sleepyhead, are you finally awake?
new
'neu' and 'alt' are often opposites. 'neu' describes something that has only existed for a short time, has hardly been used, or means a change. 'alt' describes something that has existed for a long time, is used, or comes from an earlier time. For people, 'alt' refers to age; 'neu' is more appropriate for an unfamiliar person or someone who has recently joined, for example 'eine neue Kollegin' (a new colleague). With food, 'alt' can also mean that something is no longer fresh.

Beim Frühstück merkt sie, dass das Brot alt geworden ist.
At breakfast, she notices that the bread has gone stale.
Vor dem Termin zeigt mir Frau Becker das neue Formular.
Before the appointment, Ms. Becker shows me the new form.