Answer:
The state of Maryland has two laws with regards to the carry of knives. Title 4-101 refers to the concealed carry of many types of weapon, while Title 4-105 regulations the sale (but not ownership) of switchblades.
In a nutshell, folding pocket knives (called "penknives" in this law) of any size are legal to carry concealed or openly, so long as they are not a switchblade. There is no limitation based on age or the length of the blade. The lack of length limitation has been upheld in Maryland's highest court since 1978. Sometimes police officers can be ignorant of this, but they do so at their own peril. One officer in 2002 was tried and convicted of False Arrest because he arrested a man for a 3 inch long folding knife, mistakenly believing it was illegal.
Please note that some counties and cities have the right to make restrictions that are tighter than that of the state, so always check those as well.