
At what stage do boys start getting erections?

Male fetuses can experience erections as early as when they are in the womb.
How many erections does a man get on average in a day?

The average man has about 11 erections each day and several more at night
Men can orgasm without ejaculating

Some men can have an orgasm without ejaculating at all. They still have the feeling of having an ejaculation (coming) but no semen comes out. Either there is no semen produced, or the semen travels backwards into the bladder instead (via healthdirect)
Ejaculation is the same as orgasm for men

Ejaculation is the expulsion of semen from the penis whereas orgasm is a feeling of intense pleasure. A man can ejaculate without an orgasm and vice versa
Men get erections only when they are sexually aroused

Men can get random erections without even thinking about anything sexual
Are there different types of erections?

These different types of erections have been broadly categorized as psychogenic, reflexogenic and nocturnal erections
The shorter the penis, the bigger the erection

According to the NHS, Research based on the penis measurements of 2,770 men found that shorter penises increased by 86% when erect, nearly twice that of longer penises (47%).
The penis is a muscle

The penis is not a muscle which is why it can't be moved very much when hard. It's considered more of a kind of sponge that fills with blood when a man is sexually excited. (via NHS)
Penises can break!

Not like a broken bone instead, the blood vessels within the penis burst, causing painful swelling