'老了以后再去考虑吧' this sentence has apectual or modal 了?
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.