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How much strength and dex do you loose with old age? I for sure do not want to be a prisoner to mod-gear.
Your potential max (not necessarily your actual attribute) will lower by up to three points, assuming you play your character that long.  See help age.
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Also, priests can train 7 times in ART?
More than that, but you have to consider what you're buying.  Healing and counter-spells aren't affected by art.  Neither is your high-level damage spell Harm, nor even Holy Word IIRC.  What you get is less resistance to Spear of Faith, Gate, Summon, and staves/scrolls.  If the last is what you care about, staves max at 3 art, and scrolls 6.  
It's a decent stat for priests, but maxing it definitely isn't for everyone.  You may prefer hp, or even mana for healing endurance against NPCs.