Change-of-state 了: "now it's…"
助词 / particles (语气助词 了)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Sentence + 了 (sentence-final)
At the end of a sentence, 了 signals a change of situation: something has become true that wasn't before.
天冷了 = it's turned cold (it was mild before). 下雨了 = it's started raining. 我不去了 = I'm not going after all (change of plan).
This 了 has nothing to do with past tense: it works for present or future too (快冬天了).
Examples
下雪了！
Xià xuě le!It's snowing!
秋天到了，树叶黄了。
Qiūtiān dào le, shùyè huáng le.Autumn is here, the leaves have turned yellow.
天气不好，我不去爬山了。
Tiānqì bù hǎo, wǒ bú qù pá shān le.The weather's bad, so I'm not going hiking any more.
我二十五岁了。
Wǒ èrshíwǔ suì le.I'm twenty-five now.
Common mistakes
Without 了 it's a plain description: 天很冷 (it's cold) ≠ 天冷了 (it's turned cold).
With change 了, 很 usually disappears: ✓ 天冷了 is more natural than 天很冷了.