It's not the age of the paint - I have some Tamiya acrylics which are anything from 10 to 15+ years old and are still working fine.
Under certain conditions, eg. hot weather with low humidity, Tamiya acrylics will "skin over" very quickly such that when you make another brushstroke it will drag up the paint from the previous pass.
Are you using the paint straight from the bottle or are you adding thinner to it? If you have been painting straight from the bottle, try transferring a small quantity of paint to a palette and add a little thinner. Tamiya acrylic thinner is said to contain a "retarding" agent which slows the drying time, giving you more time to "work" it and allows the paint more time to "level".
If you are using a substitute (eg. alcohol) instead of Tamiya thinner, you can also add a drop or two of acrylic retarder medium (available from art supply stores) to your mix.