What was the average life span of a person in biblical times, and how old were Jesus's disciples reportedly at the time of his death?

The Patriarchal Period during the pre-Flood period, the life span approached a thousand years. (Genesis 5:5-29) The people closer to the time of Adam enjoyed greater longevity than those farther removed from it. The longest life on record is that of Methuselah, who lived 969 years. After the Flood, the human life span dropped rapidly.

Since Moses’ Time. About 3,500 years ago, Moses wrote concerning the life span:

Psalm 90:10

10 The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.

LIFE SPANS OF THE PATRIARCHS - in the Old Testament which are chronicled in Genesis.

Adam - Genesis 5:5 - 930 years

Seth 5:8 912

Enosh 5:11 905

Kenan 5:14 910

Mahalalel 5:17 895

Jared 5:20 962

Enoch 5:23 365

Methuselah 5:27 969

Lamech 5:31 777

Noah 9:29 950

Shem 11:10, 11 600

Arpachshad 11:12, 13 438

Shelah 11:14, 15 433

Eber 11:16, 17 464

Peleg 11:18, 19 239

Reu 11:20, 21 239

Serug 11:22, 23 230

Nahor 11:24, 25 148

Terah 11:32 205

Abraham 25:7 175

Isaac 35:28 180

Jacob 47:28 147

The average of these years is 413.

To the second part of your question, the Bible does not say explicitly what the ages of the disciples were, but there are a few hints that at least some of them were relatively young — probably late teenagers or in their early 20s.

In Jewish tradition, a young man began following a Rabbi between the ages of 12 and 30, and usually when he was less than 20.

Another relevant fact is that John lived until at least AD96 when Revelation was written, which is 66 years after Jesus died. That suggests that John was almost certainly a teenager when he joined Jesus.

There is an account where Jesus and Peter pay the temple tax which was required to be paid by every man aged 20 years and older.

Exodus 30:13-14

13 This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel is twenty gerahs:) an half shekel shall be the offering of the Lord.

14 Every one that passeth among them that are numbered, from twenty years old and above, shall give an offering unto the Lord.

The other disciples were there (see preceding verses), but do not seem to have paid the tax. Perhaps they were young enough to be exempt from paying it, and so were still teenagers.

Matthew 17:24-27

24 And when they were come to Capernaum, they that received tribute money came to Peter, and said, Doth not your master pay tribute?

25 He saith, Yes. And when he was come into the house, Jesus prevented him, saying, What thinkest thou, Simon? of whom do the kings of the earth take custom or tribute? of their own children, or of strangers?

26 Peter saith unto him, Of strangers. Jesus saith unto him, Then are the children free.

27 Notwithstanding, lest we should offend them, go thou to the sea, and cast an hook, and take up the fish that first cometh up; and when thou hast opened his mouth, thou shalt find a piece of money: that take, and give unto them for me and thee.

We know that a few of the disciples had established jobs including being a fisherman (Peter, Andrew, James and John) or tax-collector (Matthew). But as Jewish schooling normally finished at age 12, there would have been time for them to learn these trades and still join Jesus at a young age.

Respectively, Jewish tradition states that at age 5 reading the Bible, 10 learning the Mishna, at 13 bound to the commandments, at 15 the study of the Talmud, at 18 marriage, at 20 the pursuit of business.

In all this, it is hard to determine the actual ages from a biblical perspective of the disciples. Any estimate is based on the culture at the time.