As you certainly know, the key difference in the language is that between is and tá, also called “the copula” and “the substantive verb”. However, so as to speak and write good Irish, you need to grasp and master it.
As we should know by now, you can’t use tá to link two nouns, if you don’t put something between them. However, there is one important exception. You are allowed and indeed encouraged to use tá if you are telling how long, how big, how old etc. something or someone is. Thus, look at this:
Tá Seán trí bliana déag d’aois. “Seán is thirteen years old.”
There is no prepositional construction, no nothing, between Seán and trí bliana déag. Seán is thirteen years and you use tá to tell it.
Note that the trí bliana déag is followed by d’aois, i.e. “of age”. You either use ar or de before the abstract noun in this construction. It is a question of idiom whether it is ar or de. For instance, with fad and leithead it is ar (which does not lenite here: ar fad).
The formula is:
TÁ + SUBJECT + NUMBER + UNIT OF MEASUREMENT + DE/AR + ABSTRACT NOUN (QUALITY MEASURED).