My three cents.
Age impacts second language teaching strategies with regard to two important aspects:
1. The ability of the learner to imitate language related sounds. I believe humans lose that ability at the age of between 4 and 5 years. Children before the watershed should be exposed to the foreign native speech (ideally their peer's speech) as much as possible. After the watershed this technique gives way to other methods, but once the early opportunity to learn/teach “by ear” is missed or not taken full advantage of, the chance to learn/teach to ever speak the target language without a foreign accent is irredemably lost.
2. The ability of the learner to write and read which typically coincides with an age of 5 or 6.
3 . Eagerness of the learner to ... learn. Leaving apart scientists or people with scientificic minds (in whom the eagerness to learn may actually increase with age), people tend to lose their thirst for knowledge between their 20th and 30th year. Oscar Wilde recognized the fact in this addage, "I am no longer young enough to know everything" which in the context of my train of thought could be modified thus, "I am no longer young enough to know everything so what is the point in trying". The degree of eagerness in learners to learn should be identified by teachers and acted upon accordingly.
I personally don't subscribe to the idea of ageing (save for age related illnesses) having an impact upon human brain's ability to learn. Being bombarded with huge amount of information our brains defend themselves against overloading by being selective in what is absorbed by them. I not so long ago saw a film about a number of people in whom this process of selection is morbidly diminished. Their brains record indiscriminately and involuntarily all information they are exposed to.
5jj's admitted inability to master more than a few hundred Czech words is in my opinion down to nothing else but insufficient motivation. Konungursvia’s suggestion how to improve on one’s ability to remember words by comparing their forms in different languages of a common genealogy should come in handy to 5jj in adding that scientific touch to the latter’s pursuits.