When is it too late to ask for salvation?

Hebrews 9:27 is often taken to indicate that judgement directly follows death, so that there are no more chances to repent after death:

And just as each person is destined to die once and after that comes judgment, so also Christ died once for all time as a sacrifice to take away the sins of many people. (Hebrews 9:27-28, NLT)

It's not indisputable, but it's good enough for me.

This is an old question, but I feel it's important to note that there are, in fact, some people that cannot be saved.

Matthew 12:31-32 tells us that every sin and blasphemy is forgiven, however someone who commits blasphemy against the Holy Spirit will not be forgiven.

"Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy against the Holy Ghost shall not be forgiven."

We also see that Apostates (those who have fallen away) in Hebrews 6:4-8:

For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.

We are told, depending on denomination, that God will either judge us upon our death and/or at the Final Judgement in Revelation. While it may be (and I believe) that God can forgive the two types of people mentioned above, I see no reason why He would put Himself in contempt by not doing what He said (through His Word) He would/would not do, as He does not change:

Numbers 23:19 - God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good?

I'll ditto Eric. But I would add:

The Bible makes it clear that you can be saved on your deathbed, e.g. the theif on the cross (not exactly a death BED, but whatever). But Jesus told several parables in which he made clear that there is some point at which it will be too late. For example, read the parable of the "wise and foolish virgins" in Matthew 25:1-13. We could debate just when that point is, but there is some point.

Baptists routinely say, "there is no second chance", by which they mean, no chance after death. I don't know of anything in the Bible that explicitly says this, but it's not inconsistent with the combination of accounts of deathbed conversions and warnings about waiting until it's too late. The story of "the Rich Fool" in Luke 12:13-21 could be taken to mean that it's too late once you're dead.

"If we are baptized once for the forgiveness of sin, does it make less sense to do it at a young age (more chances to screw up)?" It's not like baptism takes care of all the sins you committed before you were baptized but you're expected to be perfect after that. Unless you die within minutes of being baptized, you will almost surely sin again afterwards. Traditional Christian teaching is that Jesus death pays the price of ALL the sins of the saved, not just the sins they commit before baptism.

There is an old saying (I believe from Dante) that "the gates of Hell are locked from the inside." In The Great Divorce by C.S. Lewis, this is reiterated by a solid one who says:

"In the end, there are only two kinds of people - those who will say to God, 'Thy will be done,' and those to whom God must reluctantly say, 'Your will be done.'"

The idea is that hell is a choice. In this understanding, Hell is the place where God's presence is either not felt at all, or is as weak as possible. Hell is an attempt to avoid the reality of God. In such an understanding, it is not that God rejects those in hell, but rather that those in hell reject God.

When God hardened Pharaoh's heart, for example, it was Pharaoh who resisted God. God merely strengthened the choice Pharaoh had made, and drew him to its natural conclusion - death. If hell operates like this, then death is not so much a finality of choice as it is a point of no return. It is not a point after which it is impossible to change one's mind, but it is a point after which it would become much harder and much more rare.

In the same way that most people come to Christ in their teens and twenties (and back to Christ after their first child), it is not a preclusion that makes it impossible for someone in their seventies or eighties to do so - but the numbers bear me out on the fact that its not an everyday sort of thing...

Considering that the thief on the cross asked and was forgiven as he was dying on a cross near Jesus I'd have to argue that there is never a moment that is too late until you are dead.

I don't know of any teachings in my sect (reformed protestant, specifically Presbyterian) that either allow or prohibit the idea of conversion after death. However, because of the way that we understand sotieriology and the sovereignty of God it seems unlikely.

Basically out theology of salvation/sovereignty is summed up in Romans 8:29-30

29 For those God foreknew he also predestined to be conformed to the image of his Son, that he might be the firstborn among many brothers and sisters. 30 And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified.

In short God has predestined his people and calls them. The Presbyterians teach that this calling is irresistible and permanent. Thus, God will call his people to himself when he pleases (even after death, if it pleases him (although that is pure opinion on my part)).

On your final question, the Presbyterian understanding of Baptism is a bit different, it is a sign of the covenant rather than a saving act for salvation. We believe that the baptism that is referred to in your inference is the baptism of the Holy Spirit, is permanent and only happens once. Because God is outside of time he forgives all of your sins past and present at the time of your acceptance of Christ (and by extension, this baptism of the spirit). The age at which you receive this baptism is irrelevant as it is effective for all your sins, not only the ones you have done in the past.