Defective gear shifting down My mountain bicycle has 3 chainrings at the front and 7 cogs at the back, making it a 21 gears bike, which are shifted by a derailleur system.

Although it is 25 years old, everything seems to work fine, except for switching gears down from 3-7 to 3-6 and from 3-6 to 3-5. (With X-Y I mean Xth ring, Yth cog.) Whenever I try one of these two particular shifts, they take longer to kick in and in the meantime I hears "clicks" from the mechanism, differently from the other shifts, which take effect fast and smoothly.

The same behavior also happens in another 21 gears bicycle I sometimes ride, which leads me to the question:

Is this effect widespread and normal or is it a malfunction I'd better repair?
It's reasonably common to have some shifting problems with derailleur systems, especially on a 25 year old bike.

A common problem is friction between the shifter cables and housing due to wear and dirt having worked it's way in. What is probably happening in your case is that the cable is 'hanging up' in the housing delaying the movement of the derailleur. The clicking is the chain hitting the adjacent sprocket until the derailleur eventually moves over to it's proper position.

Replacing the housing and cable is relatively cheap, will make shifting more accurate and reduce the force needed to change gear. Replacing cables and housing is quite easy and there are many video guides available.

A worn or dirty chain can also cause or exacerbate problems, as can wear to the rear sprockets.