I'm a student of Chinese. I know the differences about modal and aspectual 了，but this sentence, 老了以后再去考虑吧 confuses me. I think is not modal, because modal should be at the end of the sentence, but a the same time 老 is not a verb, so I can't understand if it is aspectual or modal?
老 can be a verb for "to age/ to get old"
了 in 老了 is an aspect marker that indicates the verb/ action is completed, therefore , 老了 means "aged/ gotten old "
[老了以后]再去考虑吧] --> [after you've gotten old], then think about it
Examples of 老 as a verb:
我老了 = I am getting old
看似老了幾年 = seem like aged a few years
Note: 了 only indicate completion of a verb, it doesn't indicate tense.