As regards to upper limits, there is little doubt that matters become more difficult the older one gets. Mastering a foreign tongue, though, is far from impossible. At 45 the linguist Chao Yuen-ren went to Turkey and after two months could converse easily with the local people.
但這也有例外，像民初史學大師陳寅恪三十幾歲開始研究梵文，四十多歲卓然有成，語言學家趙元任四十五歲遊土耳其「不出二個月，就能與當地土著交談」，即使在今日台北，師大國語教學中心六、七十歲以上的老學生也還是很多。