no different ,the both are same mean

same... 你 多大 = 你 多大 了

Not only in "Street language". It is known that there is definite Inflectional affixes like "ing" or "ed" following verbs in English or other most languages. In Chinese, we express something is done using “了 le ”， like these "我做完了，我吃完了，他写完作业了，他们看完电影了....",and express something is being done by somebody using "着 zhe”, like these "我玩着呢！ 他打球着呢！他们等着别人呢！"

了 is more used in "the street language" i'd say

1. 了 is using in the end of sentence to express passed time 我吃过了，means I have eaten up 2. 你多大了 here is expressed stress of speaking without any other things