Starému člověku v tomto věku už postupně ubývají síly.

An old man is already gradually getting/gets/ (losing/loses strenght) weaker at this age.

An old man has been already gradually getting weaker/losing strenght at this age.

An old man has already gradually got weaker at this age.

An old man's strenght already gradually gives out/slackens/sub­sides at this age.

An old man's strenght has gradually given out/slackened/sub­sided at this age.

An old man's strenght has been gradually giving out/slackenin­g/subsiding at this age.

• potřeboval bych pomoci s tímto doslovným překladem (s přesným zachováním slov už ,postupně…) se správným výběrem slov a hodí se více předpřítomný čas, když se to děje celý život postupně nebo stačí přítomný čas?,díky.

Without the wider context, it's not clear if you're talking about one particular old man, or old people in general, or whether the fact that it's a man or a woman is important here. “Elderly” is better than “old”.

Again, without the context, I assume that the age has already been mentioned, so it's that [= what I've just told you], not this [what I'm going to tell you].

• The elderly gradually become weaker at that age.
• An elderly man gradually becomes weaker at that age.
• The elderly gradually (begin to) lose their strength at that age.
• An elderly man gradually begins to lose / loses his strength at that age.

Again without context, it's not clear where the emphasis is in the sentence. Maybe:

• That's an age when the elderly gradually lose their strength.

Present perfect is also possible (“the elderly have gradually become weaker at that age”) but this depends on the tenses and time references of the rest of the narrative.

děkuji moc, bylo to jinak obecně muži/ženy v důchodovém věku atd.