Its not "weakness" due to masturbation. as one ages one needs more stimulation to get erections hence penis appears more sleepy or non erect even on starting sexual act. I will give one analogy. a child may get excited seeing a chocolate in his early days. But as he grows he wants more things to excite him similarly as teenagers men get excited very fast. But as they get sex experience ( through masturbation or otherwise or see porn ) the amount of stimulation which they need to get an erection is increased. Hence when guys above 25 get aroused, they get semi erect and it becomes fully erect only after real person touches or sex act begins. This is normal.